A method of detecting an image of a reflective object includes the steps of capturing an exposure image by an image sensor while an LED is ON; down-sampling an exposure image signal from the image sensor to a first digital signal by comparing the level of the first image signal with a first reference level; capturing a dark image by the image sensor while the LED is OFF; down-sampling a second image signal from the image sensor to a second digital signal by comparing the level of the second image signal with a second reference level; computing an image of difference between the exposure image and the dark image by subtracting the second digital signal from the first digital signal; and specifying the image of the reflective object by locating a bright point remaining in the image of difference.