1. A peripheral monitoring device for a working machine, comprising: imaging means that acquires an image of periphery of the working machine; obstacle detecting means that detects obstacles present around the working machine; position calculating means that calculates positions of each of the detected obstacles relative to the working machine; hazard zone calculating means that calculates a hazard zone around the working machine, based upon at least one of an attitude and motion of the working machine; determining means that determines, by the calculated positions of the obstacles, whether further obstacles are present in the calculated hazard zone; risk level setting means that sets, for any obstacles determined by the determining means to be present in the hazard zone, a contact risk level based upon at least one of the type, position, and height of each obstacle; image generating means that converts the acquired image into a bird's-eye image obtained when the acquired image is viewed from a bird's-eye visual point set above the working machine, and then generates an image in which a position of the bird's-eye visual point in the bird's-eye image is changed according to the contact risk level set by the risk level setting means; and display means that displays the generated image.