To develop both a lock function and an alarm function without impairing aesthetic appearance while providing an effect for making it hard for a lock plate to be viewed from the outside by surface-adhering the lock plate having the locking function and the alarm function to the horizontal frame of an outdoor side sliding door by utilizing the step between the vertical frame and the horizontal frame of the outdoor side sliding door.

The lock state of an indoor side sliding door 2 to the door end surface 6 of a door end side vertical frame 3 is formed by erecting a lock piece 8 installed on a fixed substrate surface-adhered to the door end side inner surface of the horizontal frame 4 of the outdoor side sliding door 1. The lock piece 8 is pressed against the door end surface 6 of the indoor side sliding door 2 while keeping a locked state and slides laterally when both sliding doors 1, 2 are slid relative to each other. A switch installed on the fixed substrate is displaced to a normally closed or normally open position to bring an electric circuit for alarm formed on the fixed substrate into a normally alarm operating state.