A replenishment unit replenishes a storage unit with toner. A detection unit detects a toner density of developer in the storage unit. A stoppage unit stops, based on the toner density, an image forming operation. A first calculation unit controls the detection unit to detect the toner density in a duration from when the operation stops to when the image forming unit resumes the operation, and calculates a difference between the toner density and a target. A second calculation unit accumulates the difference to obtain a cumulative value. A determination unit determines a value for determining whether or not replenishment of the toner is required, based on the difference calculated and the cumulative value calculated. A controller controls a timing at which the replenishment unit replenishes the storage unit with the toner in the duration based on the value.