1. A side-monitoring apparatus for a motor vehicle comprising: image pick-up means mounted in the motor vehicle for picking up an image in a side direction of the vehicle; approaching object detecting means for detecting approaching degree of an approaching object which approaches a driver's own vehicle from a side direction road crossing his own vehicle on the basis of two images picked up at two timings apart by a prescribed interval by said image pick-up means; and danger decision means for deciding a danger on the basis of said approaching degree.