A printer includes an image forming unit to form a toner image on an image receptor by using a toner containing fluorescent additives for generating light in a predefined wavelength band in response to light in an invisible wavelength band, transfer the toner image to a print medium, and fix the toner image on the print medium, a photodetector including a light emitting unit configured to irradiate light in an invisible wavelength band to the toner image accommodated in one of the image receptor and the print medium, and a light receiving unit to detect reflected light from the toner image, and a controller to control a printing operation of the image forming unit, based on a detection signal from the light receiving unit.