Imaging device picks up an image of the area around a vehicle, edge extraction function extracts an edge image from the image picked up by the imaging device, edge width standardization function standardizes an edge width of the edge image extracted by the edge extraction function, counting functions increments a count value corresponding to a position where the edge image standardized by the edge width standardization function is detected, and also initializes a count value corresponding to a position where the standardized edge image is not detected, moving speed detection function calculates a moving direction and moving speed of the edge image extracted by the edge extraction function based on the inclination of the count values, and collision time calculation function calculates the time of collision with an object by utilizing the position and the moving speed of the edge image calculated by the moving speed detection function.