A slider is mounted on a housing of one of male and female connectors to be fitted together, and is urged in one direction by a spring, and flexible arms and a lock arm are formed integrally with this slider. Second engagement projections, formed on a housing of the female connector, push the flexible arms, respectively, and are engaged respectively with the flexible arms when the two connectors are fitted together. A retaining projection for retaining the lock arm is formed on that housing. In a half-fitted condition, the second engagement projections are not engaged respectively with the flexible arms, and the female connector is pushed back by the spring, so that the half-fitted condition can be detected. The lock arm is engaged with the retaining projection, and also the flexible arms are engaged respectively with the second engagement projections, thereby preventing the two connectors from being accidentally disengaged from each other.