Patent ID: 8861992

Claim:
An image forming device comprising: a replaceable cartridge accommodating developer therein and including: an agitator rotatable and configured to agitate developer accommodated in the replaceable cartridge; and a developing roller configured to carry the developer thereon and rotatable together with the agitator; a light-emitting element configured to emit a light toward the replaceable cartridge; a light-receiving element configured to receive the light emitted by the light-emitting element through the replaceable cartridge to output a signal, a determination value being produced based on the signal; a determining unit configured to determine whether or not the determination value is greater than a predetermined determination threshold upon the agitator having rotated continuously more than a first prescribed number of times and determine that the replaceable cartridge should be replaced with a new one when the determination value is greater than the predetermined determination threshold; a counting unit configured to count an index value indicating quantity of developer used while a print job is consecutively inputted a predetermined number of times, an image formation for each of the predetermined number of print jobs being completed before the agitator rotates a second prescribed number of times continuously, the second prescribed number being smaller than or equal to the first prescribed number; and a control unit configured to control the agitator to rotate continuously more than the first prescribed number of times if the index value exceeds a prescribed index threshold.