To positively detect whether a person is sitting on the seat of an automobile or not by arranging an ultrasonic conductor below a seat cover or a carpet and discriminating the attenuation state of ultrasonic this is generated by a ultrasonic vibration means using an ultrasonic detection means and a discriminating means.

An ultrasonic conductor 2 for propagating ultrasonic vibration such as a metal or ceramic is arranged, for example, below the seat cover of a passenger car or below a carpet, and one edge is connected to an ultrasonic vibration means 1. An ultrasonic detection means 3 is connected to the ultrasonic conductor 2 and is converted to an electrical signal while the propagated ultrasonic vibration is amplified. A discriminating means 4 detects the attenuation of ultrasonic vibration due to the propagation of the ultrasonic conductor 2 from an electrical signal received from the ultrasonic detection means 3 and judges whether the amount of attenuation is large or small. When the amount of attenuation is large and an amplitude is small, the ultrasonic conductor 2 is suppressed by a large force, thus judging a person is sitting on the seat.