When two connector housings are half-fitted together, a fitting detecting connector causes one of the connector housings to be pushed in a direction of separation by means of springs which are provided within a female connector housing, a spring holder being inserted into this female connector housing so that it can move in an anterior-posterior direction. This spring holder houses springs and, when the two connector housings are being fitted together, a locking arm rises over a stopping protrusion. As a result of the locking arm rising, the spring holder is restrained, its movement in a posterior direction is regulated, and the springs are compressed. When the connector housings are completely fitted together the locking arm springs down, releasing the spring holder, and allowing it to move in a posterior direction under the spring force. All of the half-fitting detector components are provided on one of the connector housings; the other connector housing requires no modification.