To solve such a problem that alarm may not be recognized by being disturbed by game effect sound, BGM, and electric lamps, etc., though a means for issuing the alarm when a front door is opened is provided as measures against the fraudulent opening of the front door, and also a scene with a fraudulent action is not confirmed since time elapses after issuing the alarm even when the alarm is recognized.

The height of a chair for a game machine, which is arranged in each game machine, i.e., the height of the seat surface is locked at an abnormally low position which is abnormally lower than a normal height position, based on a front door opening detection signal. Consequently, a person in charge immediately and visually confirms the fraudulent opening of the front door.