There is provided an image forming apparatus that is capable of reducing work burdens imposed on a user in terms of toner cartridge replacement. When a cartridge detection sensor detects that at least one of toner cartridges is emptied, a specified amount of toner is forcibly supplied from another toner cartridge than the toner cartridge subjected to detection to an intermediate hopper. After that, the amount of the toner within the another toner cartridge is detected. When an empty toner cartridge is newly detected, a request for replacement is issued as to both of the newly detected empty toner cartridge and the previously detected empty toner cartridge.