To easily detect seating of an occupant with a simple structure by mounting a variable resistance pressure detecting element having a mounting hole in the center between a seat mounting part and a floor surface through a hole.

A pressure detecting element 1 such as variable resistance rubber or the like has a disc-like flat form, and it has a mounting hole 1a provided in the center part so as to vertically extend through. Terminals 2, 3 nipping it from above and under have mounting holes 2a, 3a of the same form, and insulating plates 4, 5 arranged on the outside thereof similarly have mounting holes 4a, 5a to nip the terminal plates 2, 3 from above and under. Such pressure sensors A are mounted between mounting parts in four corners of a seat and a floor surface through the mounting holes 1a-5a. When an occupant sits on a seat, the weight is dispersed to the pressure sensors in the four corners, and an adder circuit adds the converted weight signals and outputs an added signal. An occupant detecting judging circuit judges that a person is sitting to turn on a switch, when the added signal exceeds a reference value, for example, 35 kg, and also supplies an ignition current to a detonator for the seat next to the driver as occasion demands.