To stably and accurately rotate a developer carrier not only initially but also after the lapse of time in a device for transmitting driving to the developer carrier in an image forming apparatus.

A rotary developing device 14 is provided with four developing units 22. Each developing unit is equipped with a developing roller (developer carrier) 25, and a magnet attaching part 52a is formed on the shaft 52 of the roller 25, and a magnet 53 is fixed by adhesion on the end face of the magnet attaching part of the developing unit existing at a developing position. The magnet constitutes a magnetic coupling 55 with a pair of magnets 54 opposed through a void (s). A pair of magnets 54 is fixed on the rotary shaft 57 of a driving unit 56 properly supported in a copying machine device main body 10. Thus, rotational force from the driving unit being a driving source is transmitted to the roller 25 through the magnetic coupling 55.