A recording material characteristic detecting device that irradiates a first light and a second light to a recording material carried along a predetermined carrying path and detects a transmitted light generated when the first light transmits the recording material and a reflection light generated when the second light is reflected by the recording material, and detects a characteristic of the recording material, comprises: a reflection light source that irradiates the second light toward the carrying path; a reference plate for reflection light arranged oppositely to the reflection light source across the carrying path to reflect the second light; and a light detector arranged oppositely to the reference plate for reflection light across the carrying path to detect at least a reflection light generated in response to reflection of the second light. The reference plate has a higher reflectivity to the second light than a reflectivity to the first light.