1. An image forming apparatus comprising: an image bearing member; image forming means for forming an electrostatic image on said image bearing member in accordance with an image signal; developing means for developing an electrostatic image formed on said image bearing member; transfer means for transferring a developed image on said image bearing member onto a transfer material; speed setting means for selectively setting a moving speed of said image bearing member; and gamma characteristic selection means for selecting a gamma conversion characteristic relationship between the image signal and an output density in accordance with the moving speed of said image bearing member set by said speed setting means.