A support unit supports a rotation axe of each of at least three latent-image carriers in a rotatable manner. A plurality of developing units develops latent images on the latent-image carriers with toners of different colors, respectively. A toner-pattern detecting unit detects a toner pattern formed on a belt member that is suspended by a plurality of suspending members. A displacing unit is provided to the support unit to displace the rotation axis along a surface of the belt member in a direction of movement of the surface of the belt member. A control unit controls the displacing unit based of a result of detecting the toner pattern by the toner-pattern detecting unit.