To provide a connector that prevents a lock mechanism from being broken in an operation for fitting housings of connectors.

When a female side housing 1 is fitted with a male side housing 3, insertion of fingers of an operator into a space in a flexion direction of a lock arm 11 can be prevented by functions of a restraining protrusion 15 and side ribs 16 of the female side housing 1. Thereby, while flexion of the lock arm 11 is disturbed, the female housing 1 is not fitted with the male side housing 3, and therefore, loss of the lock mechanism due to breakage of a projection part 13 of the lock arm 11 after the female side housing 1 is regularly fitted with the male side housing 3 can be prevented. Also, the side ribs 16 are arranged on both sides of the lock arm 11, so as to restrain the operator from performing a fitting operation while unintentionally flexing the lock arm 11. Therefore, an inertia lock mechanism can be reliably provided.