A collision avoidance assisting system for a vehicle, for expecting a risk of colliding upon a moving object (or a moving obstacle), including a pedestrian, more correctly, but without annoying a driver, excessively, by estimating the risk to be excessively high, comprises a moving object detecting means for detecting a moving object existing on periphery of the vehicle; a footway boundary detecting means for detecting a position and a configuration of a footway boundary object on periphery of the vehicle; a risk estimation means for estimating a risk that the moving object detected by said moving object detecting means collides on the vehicle; and an alarm means for calling an attention to a driver of the vehicle, upon basis of the risk of collision estimated by the risk estimation means, wherein the risk of collision between the moving object, which is detected by the moving object detecting means, and that vehicle is estimated by taking at least the position information of the moving object, the position information of the footway boundary object and conditions of the circumferences thereof.