PURPOSE: To check male and female housings for incomplete coupling as well as double locking, and prevent the occurrence of an incomplete coupling condition.

CONSTITUTION: When a coil spring 15 is mounted on the spring retaining pin 13 of a slide lock C and inserted into a lock chamber 7, the stopper 14a of the slide lock C comes in contact with the stopper 10a of the lock chamber 7, thereby deflecting a flexible retaining piece 10 outward. When the stopper 14a goes over the stopper 10a, the slide lock C is temporarily locked. Then, a male connector housing A is coupled to the hood 6 of a female connector housing B. If the coupling condition is complete, the retaining projection 5a of the locking arm 5 of the male connector housing A comes to be engaged with a hooking hole 6a in the hood 6, thereby ensuring a primary locking condition. In the aforesaid state, when the slide lock C is pushed forward against the resilient force of the coil spring 15, a lock plate 12 advances and the hooking hole 12a comes to be engaged with the pressing part 5b of the locking arm 5. As a result, a secondary locking condition is obtained and, therefore, both housings A and B are double locked.