1. An identity authentication system comprising: a detecting unit that detects an identity theft by determining whether a photographing target is a living body or a non-living body; a collating unit that performs identity collation based on a photographed image; and a control unit that controls execution timing of a detection process performed by the detecting unit and an identity collating processing performed by the collating unit and, in a case where the detection performed by the detecting unit is performed for a first number of times, performs the collation process performed by the collating unit, wherein the first number of times is set in consideration of a tradeoff between a required intensity of security and convenience of a user using the identity authentication system.