A record medium determining device 1, which determines a kind of a record medium 16 by irradiating a laser light 17 to a surface of the record medium 16, detecting a received light position and a received light intensity of a reflected light 18 by a line sensor 14, and comparing a distribution state of the received light position and the received light intensity with a previously determined distribution state, is disposed on the upstream side of a record medium conveyance path 653 with respect to a fixing unit.