To provide a recording material determination device in an image forming device capable of enhancing the determination accuracy of the kind of a recording paper or the like.

A recording paper determination sensor 100 comprises an LED 101 as a first light emission means, a photo-transistor 103 as a first light reception means, and a photo-transistor 102 as a second light reception means. The kind of a recording paper is determined by comparing the output of the first light reception unit 101 and the output of the second light reception unit 102 with the predetermined threshold. When the relative difference between the result of detection and the threshold is within a predetermined range, the position of detection of the recording paper is changed and the quantity of the reflected light is monitored for a plurality of times to determine the kind of the recording paper. When the relative difference between the result of detection and the threshold is not within the predetermined range, the kind of the recording paper is determined by using the result of detection.