A locking arm 27 is supported by a fulcrum 31 attached to a body 28 and a pair of coupling portions 33, 34. When an operation portion 30 is pressed down, an engaging portion 29 located between the fulcrum 31 and the coupling portions 33, 34 is shifted vertically to a direction of coupling a connector 10 with a complementary connector 38. The coupling portions 33, 34 are made in parallel to the direction of coupling the connector 10 with the complementary connector 38. The pair of coupling portions 33 and 34 are formed as a pair of legs. A sliding groove 35 is formed between the pair of legs 33 and 34 so as to correspond to an engaging portion 44 of the complementary connector 38. The engaging portion 44 will be slid into the sliding groove 35. In such a configuration, the locking mechanism can remove backlash between a female connector and a male connector to provide high coupling accuracy.