An image forming apparatus includes an improved structure to prevent a user from coupling expendable supplies in incorrect positions upon replacement, and a power coupling device is mounted in the image forming apparatus as the improved structure. The power coupling device includes a plurality of driving couplers provided at a drive unit, and a plurality of driven couplers provided at a driven structure to correspond to the driving couplers, respectively. Each of the driving couplers has a first coupling portion to be coupled with a corresponding one of the driven couplers, and each of the driven couplers has a second coupling portion having a configuration corresponding to that of the first coupling portion. At least two of the first coupling portions have different cross sections from each other. The driven structure, adapted to receive power of the drive unit, includes a toner cartridge, photosensitive member, etc. provided in a developing unit of the image forming apparatus.