1. An identity authentication system comprising: a detector that detects an identity theft by determining whether a photographing target is a living body or a non-living body; a collator that performs identity collation based on a photographed image; and a controller that limits execution timing of a detection process performed by the detector and an identity collating processing performed by the collator; and in response to the determination that the detection performed by the detector is performed for a first number of times, performs the collation process performed by the collator; the controller further performs: uses the first number of times that is set in accordance with a predetermined load level applied to the identity authentication system relating to convenience; gradually changes the first number of times to approach a number of times that is set in accordance with a required intensity of security, in accordance with an accumulated number of times of detection of an identity theft performed by the detector; in response to the detector detects an identity theft, causes the collating unit to perform identity collation; in response to the determination that identity collation corresponding to a second number of times is successful within a predetermined time, determines that identity verification is made; in response to determination that the intensity of the security is raised, increases the first number of times, the second number of times, or the predetermined time; and in response to determination that the convenience is improved, decreases the first number of times, the second number of times, or the predetermined time.