diff --git a/python/src/tokenizer.rs b/python/src/tokenizer.rs
index 525c674a..2d6ebb51 100644
--- a/python/src/tokenizer.rs
+++ b/python/src/tokenizer.rs
@@ -136,6 +136,10 @@ impl PyTokenizer {
     /// :param out: tokenization results will be written into this MorphemeList, a new one will be created instead.
     ///    See https://worksapplications.github.io/sudachi.rs/python/topics/out_param.html for details.
     ///
+    /// A Tokenizer instance cannot be used concurrently from multiple threads.
+    /// For parallel tokenization, share a Dictionary and create one Tokenizer
+    /// per worker/thread. Concurrent calls raise sudachipy.errors.SudachiError.
+    ///
     /// :type text: str
     /// :type mode: SplitMode | str | None
     /// :type out: MorphemeList
@@ -145,7 +149,7 @@ impl PyTokenizer {
     )]
     #[allow(unused_variables)]
     fn tokenize<'py>(
-        &'py mut self,
+        self_: &Bound<'py, Self>,
         py: Python<'py>,
         text: &'py str,
         mode: Option<&Bound<'py, PyAny>>,
@@ -157,8 +161,19 @@ impl PyTokenizer {
             None => None,
             Some(m) => Some(extract_mode(m)?),
         };
-        let default_mode = mode.map(|m| self.tokenizer.set_mode(m));
-        let mut tokenizer = scopeguard::guard(&mut self.tokenizer, |t| {
+
+        let mut this = match self_.try_borrow_mut() {
+            Ok(this) => this,
+            Err(_) => {
+                return errors::wrap(Err(
+                    "Tokenizer is already in use. A Tokenizer instance cannot be used concurrently; create a separate Tokenizer per thread or guard calls externally",
+                ))
+            }
+        };
+
+        let projection = this.projection.clone();
+        let default_mode = mode.map(|m| this.tokenizer.set_mode(m));
+        let mut tokenizer = scopeguard::guard(&mut this.tokenizer, |t| {
             default_mode.map(|m| t.set_mode(m));
         });
 
@@ -175,8 +190,7 @@ impl PyTokenizer {
             None => {
                 let dict = tokenizer.dict_clone();
                 let morphemes = MorphemeList::empty(dict);
-                let wrapper =
-                    PyMorphemeListWrapper::from_components(morphemes, self.projection.clone());
+                let wrapper = PyMorphemeListWrapper::from_components(morphemes, projection);
                 Bound::new(py, wrapper)?
             }
             Some(list) => list,
diff --git a/python/tests/test_tokenizer.py b/python/tests/test_tokenizer.py
index 623cb78c..93314550 100644
--- a/python/tests/test_tokenizer.py
+++ b/python/tests/test_tokenizer.py
@@ -13,9 +13,12 @@
 # limitations under the License.
 
 import os
+import queue
+import threading
 import unittest
 
 from sudachipy import Dictionary, SplitMode
+from sudachipy.errors import SudachiError
 
 
 class TestTokenizer(unittest.TestCase):
@@ -166,6 +169,77 @@ def test_tokenizer_out_param(self):
         self.assertEqual(id(ms1), id(ms2))
         self.assertEqual(m.surface(), 'すだち')
 
+    def test_concurrent_tokenize_on_same_tokenizer_fails(self):
+        text = '東京都庁に行きました。' * 1000
+        ready = threading.Barrier(8)
+        errors = queue.Queue()
+        unexpected = queue.Queue()
+
+        def worker():
+            try:
+                ready.wait(timeout=10)
+                for _ in range(20):
+                    self.tokenizer_obj.tokenize(text)
+            except SudachiError as err:
+                errors.put(str(err))
+            except Exception as err:
+                unexpected.put(err)
+
+        threads = [threading.Thread(target=worker) for _ in range(8)]
+        for thread in threads:
+            thread.start()
+        for thread in threads:
+            thread.join()
+
+        self.assertTrue(unexpected.empty(), list(unexpected.queue))
+
+        messages = list(errors.queue)
+        self.assertTrue(
+            any('Tokenizer is already in use' in message for message in messages),
+            messages,
+        )
+        self.assertFalse(
+            any('Already borrowed' in message for message in messages),
+            messages,
+        )
+
+    def test_separate_tokenizers_work_in_threads(self):
+        errors = queue.Queue()
+
+        def worker():
+            try:
+                tok = self.dict_.create()
+                for _ in range(50):
+                    morphemes = tok.tokenize('東京都庁に行きました。')
+                    self.assertGreater(morphemes.size(), 0)
+            except Exception as err:
+                errors.put(err)
+
+        threads = [threading.Thread(target=worker) for _ in range(8)]
+        for thread in threads:
+            thread.start()
+        for thread in threads:
+            thread.join()
+
+        self.assertTrue(errors.empty(), list(errors.queue))
+
+    def test_tokenizer_is_released_after_internal_error(self):
+        with self.assertRaises(SudachiError) as cm:
+            self.tokenizer_obj.tokenize('あ' * 20000)
+
+        self.assertIn('Input is too long', str(cm.exception))
+
+        morphemes = self.tokenizer_obj.tokenize('東京')
+        self.assertGreater(morphemes.size(), 0)
+
+    def test_temporary_mode_is_restored_after_internal_error(self):
+        tok = self.dict_.create(SplitMode.C)
+
+        with self.assertRaises(SudachiError):
+            tok.tokenize('あ' * 20000, SplitMode.A)
+
+        self.assertEqual(SplitMode.C, tok.mode)
+
 
 if __name__ == '__main__':
     unittest.main()