PURPOSE: To surely realize the seating detection by using a lavatory seat as a transmission medium of an ultrasonic wave, and also, to improve the design property of a Western-style stool by attaching an ultrasonic transmitter ad an ultrasonic receiver to the reverse side of the lavatory seat.

CONSTITUTION: An ultrasonic transmitter 7 and an ultrasonic receiver 8 are attached to the reverse side of a lavatory seat 3, and an ultrasonic wave transmitted from the ultrasonic transmitter 7 is transmitted by using the lavatory seat 3 as a transmission medium, and received by the ultrasonic receiver 8. Also, the ultrasonic transmitter 7 and the ultrasonic receiver 8 are connected to a transmitting/receiving circuit 11 contained in a control box 6 by lead wires 9, 10. In the case a person 14 who goes to the closet is seated on the lavatory seat 3, since the person 14 who goes to the closet comes into contact with the lavatory seat 3, an ultrasonic wave is attenuated in the process in which it is transmitted through the lavatory seat 3, therefore, amplitude of the ultrasonic wave received by the ultrasonic receiver 8 becomes small. In such a way, since a voltage level of an electric signal outputted from a circuit 13 becomes lower than a prescribed value, a seating signal (logical H signal) is outputted from a discriminating circuit 15, and seating can be detected surely.