An image forming apparatus includes an image forming device for forming a pattern image having unit images on a photosensitive member, a sensor for irradiating the pattern image with light and detecting reflected light, and a controller for controlling the image forming device to form a pattern image for printing used when printing and a pattern image for detection to be detected by the sensor on the photosensitive member. A first pattern image for printing includes a predetermined number of first unit images arranged to be spaced from each other, each of the first unit images being smaller than a predetermined size. A first pattern image for detection corresponding to the first pattern image for printing includes more than the predetermined number of the first unit images arranged to be spaced from each other.