A controller can select and execute a full color mode of performing image formation in a first state in which each primary transfer member is in contact with an intermediate transfer belt, and a monochrome mode of performing image formation in a second state in which only a primary transfer member is in contact with the intermediate transfer belt. The controller determines which state of the first state and the second state is formed, based on a detection result obtained by a current detection circuit when a photosensitive drum is charged by a charging roller, and voltage is applied to the primary transfer member from a primary transfer power source.