To provide a side edge detecting device for accurately detecting a side edge of a recording paper sheet.

This side edge detecting device is equipped with a pair of side edge sensors 27, 28 in the vicinity of a heating element array of a thermal head. Each of the side edge sensors 27, 28 consists of LEDs 30-33, light receptive windows 34a-34d, phototransistors 35-38, and amplifiers 45a, 45b. The side edge detecting device determines the side edge position of the recording paper sheet 10 based on the difference between the outputs of the phototransistors 35-38 by using a system controller 48. Influence of heat drift is eliminated so that the position of the side edge of the recording paper sheet 10 can be precisely determined. Even when the recording paper sheet 10 is shifted in the main scanning direction, it is possible to align a center of an image with the center of the recording paper sheet 10 in the main scanning direction. Disagreement of margin widths is not created so that quality of the image is improved. A heating element not facing the recording paper sheet 10 is not heated so that it is possible to suppress lowering of the life of the heating element due to idling of heating.