1. An authentication method, provided in a system having a display and a storage device, comprising the steps of: registering an object selected for each user from among a plurality of visually distinguishable objects prepared in advance as a key object in said storage device; and presenting the plurality of objects to said display, accepting selection of an object by a user to be authenticated, and performing authentication based on matching/mismatching of the selected object with the key object registered in association with the user, wherein the step of registering includes a step of determining a degree of freedom of selection of the object at the time of registration of the key object according to a degree of overlapping of the key object already registered in said storage device.