PROBLEM TO BE SOLVED: To provide an image projection system which detects a position on a screen indicated by a pointing device with a simple and inexpensive configuration.SOLUTION: An image forming system includes a projector device and a pointing device. The pointing device includes: a luminance detection unit which detects luminance of at least one pixel of a projection image; a determination unit which determines whether an amount of change in luminance detected by the luminance detection unit is larger than a threshold or not; and a transmission unit which transmits a notification signal to the projector device when the determination unit determines that the amount of change is larger than the threshold. The projector device includes: a luminance change unit which changes luminance of each of pixels included in input image data sequentially; a receiving unit which receives the notification signal; and a specification unit which specifies a position of the pixel whose luminance is to be changed, as a posi
