A high-frequency generating circuit is connected to six electrode fabrics embedded in a seat back of an assistant driver's seat. Output current flowing at the time when a human body approaches the electrode fabrics is detected. It is determined whether an occupant is an adult or a child, by comparing patterns of the output current which correspond to the six electrode fabrics with a first threshold value and a second threshold value to detect the positions of the head, neck and shoulder of the occupant. When the occupant is a child having a short sitting height, the operation of an air bag device is prohibited upon collision of a vehicle, and a warning lamp is lit to move the occupant to a rear seat.