an image forming apparatus includes a detachably - mounted cartridge including a developer accommodating portion for accommodating a developer and an operation member , a remaining amount detector for detecting a remaining amount of developer in the developer accommodating portion , an operation amount detector for detecting a cumulative operation amount of the operation member , and a setting device for setting timing of subsequent remaining amount detection of the remaining amount detector . the remaining amount detector provides a remaining amount ratio of the developer remainder to a maximum accommodation capacity of the developer accommodating portion , and the operation amount detector provides an operation amount ratio of a remaining operation amount until the end of the service life from the detection of the cumulative operation amount to a total operation amount until the end of the service life from an initial state of the operation member . the setting device sets the timing on the basis of a comparison between the remaining amount ratio and the operation amount ratio .