An image forming apparatus, which is capable of correctly and accurately detecting a sheet leading edge position and carrying out image formation in appropriate image write timing, includes a detection device disposed at a reference position upstream from the transfer position and detects the recording medium being conveyed. A determining device determines image write timing for forming the electrostatic latent image on the photosensitive drum based on a difference in position between a leading edge position of the recording medium detected by said detection device and the reference position. The detection device respectively include a plurality of light detecting pixels disposed in a recording medium conveying direction, and the leading edge position of the recording medium detected by the respective light detecting pixels.