An in-vehicle authentication device may perform wireless communication with a portable device in a vicinity of a vehicle. The in-vehicle authentication device may transmit a call signal that requests a reply of a response signal to the portable device. The in-vehicle authentication device may transmit an authentication request signal that requests a reply of an authentication signal to the portable device when the in-vehicle authentication device determines that the portable device exists in the vicinity of the vehicle by receiving the response signal. The in-vehicle authentication device may authenticate the portable device that transmits the authentication signal based on the authentication signal when the authentication signal is received.