PURPOSE: To see whether or not the door or the window of a room is locked by disposing a loop antenna around the door or the window of the room, and providing to the lock of the door or the window a condition switch comprising a reed switch, a magnetic tape and a quartz oscillator.

CONSTITUTION: A condition detection switch 6 is provided on the stator 5 side of a sash fastener 1 comprising a turning element 4 and a stator 5. The condition detection switch 6 is formed by a reed switch 6A and a magnetic tape 6B. A second antenna 18 comprising a condensor 19 and a quarts oscillator 20 etc., is provided in the upper portion of the stator 5 side of the sash fastener 2. A loop antenna formed by a solid oscillator and a condensor is disposed on the ceiling or the wall of a room. When a door or a window is locked by the sash fastener 2 the magnetic tape 6B of the condition detection switch 6 is brought into contact with the reed switch main body 6A, while when the door or the window is dislocked the magnetic tape 6B is separated therefrom. The antenna 18 and the loop antenna of the room are therefore utilized to see whether or not the door or the window is locked.