PURPOSE: To provide a side air bag controller which precisely judges the degree of side danger being due to an obstacle, and automatically develops an air bag only in a state worthy of development.

CONSTITUTION: Sensors A10 and B12 are provided in the side of a vehicle 100. An electromagnetic wave radar device having a wide detection area is used as the sensor A10, and an ultrasonic radar device which is narrow in the detection area but sharp in directivity is used as the sensor B12. Detection signals issued from the sensors A10, B12 are outputted to ECU 14, and a distance and relative speed to an obstacle are detected from the output of the sensor A10, and the size of the obstacle is detected from the output of the sensor B12. ECU 14 estimates the degree of danger being due to the obstacle from a distance to the obstacle and its relative speed and size, and automatically develops a side air bag 16 on the above degree of danger.