An image forming apparatus which includes a photosensitive member and an image forming station. The image forming station includes a charging device for charging the member, an exposure device for exposing the member to light, a developing device for developing an electrostatic latent image on the member, and a bias applying device for applying a developing bias. The image forming apparatus also includes, a transfer device for transferring a toner image to a sheet, a setting device for setting an AC voltage, an executing device for executing a test mode, a density detecting device for detecting the density of the test image, and an adjusting device for adjusting an image forming condition for the station. When the executing device executes the test mode, test images can formed at various AC voltages.