To improve an inertia locking function by a resistor arm without jeopardizing workability at the time of separating both housings.

When a resistor arm 16 collides with a collision part 35, a support part 37 comes in contact with a protruded part 21 (a free-end part) of the resistor arm 16 from a flexure space 19, and temporarily increases deflection rigidity of the resistor arm 16 with the cantilever-shape resistor arm 16 as a both-end supporting form, so that inertia force given to both housings F, M in a fitting direction directly after the resistor arm 16 is separated from the collision part 35 gets large. When the both housings F, M are separated, the protruded part of the resistor arm 16 passes by the support part 37 in a cantilever supporting state along a route further toward the flexure space 19 side than the support part 37, so that a separation resistance caused by elastic bending of the resistor arm 16 is decreased.