An image pick-up means mounted in the motor vehicle picks up a side direction of a driver's own vehicle. An approaching object detecting means detects an approaching degree of an approaching object which approaches his 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 the image pick-up means. A danger decision means decides a danger on the basis of the approaching degree. In such a configuration, a side-monitoring apparatus for a motor vehicle can accurately know the danger of collision with the object approaching from the side direction road.