The developing device has four developing units arranged along a photosensitive belt stretched across endlessly. These developing units contain toners of different colors, and each has a developing roller at its leading end which faces the photosensitive belt. The developing device has a contact-enabling mechanism for selectively bringing one of the four developing units into contact with the photosensitive belt. The contact-enabling mechanism has four cams provided such an arrangement that they are out of phase with each other by an angle of 72 degrees, thereby brining each of the developing units into contact with the photosensitive belt, one driving source for rotating the four cams and one clutch for transmitting a driving force from the driving source to the four cams so as to rotate them at the same time. With this structure, the four cams are each rotated by an angle of 72 degrees each time the clutch is set on, and one of the four developing units is selectively brought into contact with the photosensitive belt.