An in-vehicle authentication system is disclosed. The in-vehicle authentication system includes a first authentication object apparatus; and a second authentication object apparatus configured to perform communication with the first authentication object apparatus. The first authentication object apparatus and the second authentication object apparatus are configured to perform mutual authentication via the communication such that a relationship between an authenticating side and an authenticated side is exchanged, and at least one of the first authentication object apparatus and the second authentication object apparatus transmits, to a counterpart, an authentication response in response to an authentication response demand from the counterpart and an authentication response demand from itself to the counterpart by a single transmission signal.