diff --git a/seqeval/metrics/sequence_labeling.py b/seqeval/metrics/sequence_labeling.py
index b5807a3..63bbf30 100644
--- a/seqeval/metrics/sequence_labeling.py
+++ b/seqeval/metrics/sequence_labeling.py
@@ -159,15 +159,15 @@ def get_entities(seq, suffix=False):
     """
 
     def _validate_chunk(chunk, suffix):
-        if chunk in ['O', 'B', 'I', 'E', 'S']:
+        if chunk in ['O', 'B', 'I', 'E', 'S', 'M']:
             return
 
         if suffix:
-            if not chunk.endswith(('-B', '-I', '-E', '-S')):
+            if not chunk.endswith(('-B', '-I', '-E', '-S', '-M')):
                 warnings.warn('{} seems not to be NE tag.'.format(chunk))
 
         else:
-            if not chunk.startswith(('B-', 'I-', 'E-', 'S-')):
+            if not chunk.startswith(('B-', 'I-', 'E-', 'S-', 'M')):
                 warnings.warn('{} seems not to be NE tag.'.format(chunk))
 
     # for nested list
@@ -229,6 +229,12 @@ def end_of_chunk(prev_tag, tag, prev_type, type_):
         chunk_end = True
     if prev_tag == 'I' and tag == 'O':
         chunk_end = True
+    if prev_tag == 'M' and tag == 'B':
+        chunk_end = True
+    if prev_tag == 'M' and tag == 'S':
+        chunk_end = True
+    if prev_tag == 'M' and tag == 'O':
+        chunk_end = True
 
     if prev_tag != 'O' and prev_tag != '.' and prev_type != type_:
         chunk_end = True
@@ -267,6 +273,10 @@ def start_of_chunk(prev_tag, tag, prev_type, type_):
         chunk_start = True
     if prev_tag == 'O' and tag == 'I':
         chunk_start = True
+    if prev_tag == 'M' and tag == 'E':
+        chunk_start = True
+    if prev_tag == 'M' and tag == 'I':
+        chunk_start = True
 
     if tag != 'O' and tag != '.' and prev_type != type_:
         chunk_start = True