1. A recording medium imaging device comprising: an irradiation unit configured to emit light to a recording medium which is being conveyed; an imaging unit configured to capture as a surface image having a plurality of pixels, light reflected by the recording medium to which the light is emitted by the irradiation unit and which is being conveyed; and a control unit configured to determine the kind of the recording medium using the surface image captured by the imaging unit; wherein the control unit determines the kind of the recording medium using an image obtained by removing pixels which do not have predetermined brightness, from a plurality of pixels of the surface image