A rotary developing unit includes: a first developing device; a second developing device; a toner supply container that is inserted between the first developing device and the second developing device to be attached to the first developing device; a first guide formed on the toner supply container; a second guide formed on the second developing device, that is configured to slide along the first guide when the toner supply container is inserted, and to be detached from the first guide before the toner supply container is attached to the first developing device; a first position fixing unit provided at the first developing device; and a second position fixing unit provided at the toner supply container, that is configured to fix a position of the toner supply container, before the second guide of the second developing device is detached from the first guide.