Connector, connector assembly and assembling method therefor

An object of the present invention is to improve detection reliability while facilitating dimensional management of a detecting member. A detecting member 70 movable from an initial position to a detection position is mounted on a housing main body 11. The detecting member 70 includes a resilient arm portion 72 resiliently deformable in a height direction. Before the housing main body 11 is connected to a mating housing 50, the resilient arm portion 72 comes or can come into contact with a lock arm 12 from behind to prevent a forward movement at the initial position. When the housing main body 11 is properly connected to the mating housing 50, the detecting member 70 reaches the detection position where the resilient arm portion 72 enters a deformation space 25 by being pushed forward from the initial position. At the initial position, the resilient arm portion 72 applies a pre-load to the lock arm 12 in the height direction.