To confirm the presence of a trespasser from the quantity of movement of the hands of a clock by moving the hands of the clock when a door as an origin is open since a person which entering a room from a door becomes a trespasser by opening and closing the door, etc.

The oscillation part of the clock is constituted into a circuit which can have its oscillation frequency adjusted to place the clock in operation and thus a special circuit is constituted to allow the clock to operate when a switch is OFF. When a reed switch leaves a magnet, the hands of the clock move, so the presence of a trespasser can be confirmed.