1. An authentication method, provided in a system having a display and a storage device, comprising the steps of: registering an object selected for a user from among a plurality of visually distinguishable objects as a key object in said storage device, said plurality of visually distinguishable objects, from which said key object is selected for said user, being prepared in advance; after registering said key object for said user, and during an authentication process of the user, presenting a plurality of objects to said display including said registered key object associated with said user for user selection of said registered key object for authenticating said user; accepting selection of an object by the 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 said key object 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.