A detecting device and corresponding method for detecting an object in an image where the image is captured by an image pickup device mounted on a vehicle. A controller is coupled to the image pickup device. The controller extracts pixels of the image captured by the image pickup device, calculates a side direction velocity component based upon the extracted pixels, detects a road boundary based upon the calculated side direction velocity component, and detects a moving direction of the object based upon a relationship between the road boundary and the object.