Image forming apparatus and developer replacement method of image forming apparatus

A developer replacement method of an image forming apparatus including a developing unit which supplies developer to an image carrier and performs development, a supply unit which contains the developer and supplies the contained developer to the developing unit, and a temperature sensor which detects a temperature inside the image forming apparatus, the method comprising:

• calculating a first ratio of a developer supply time during which the supply unit performs supply of the developer to the developing unit and a drive time during which the developing unit is driven;
• performing developer replacing processing for discharging the developer from the developing unit and supplying the developer contained in the supply unit to the developing unit based on the first ratio and a threshold value obtained based on the detected temperature of the temperature sensor, wherein the developer comprises a toner comprising one of a decolorable toner or a non-decolorable toner;
• storing a correspondence relationship between a value of the detected temperature and the threshold value;
• and
• obtaining the threshold value from the correspondence relationship according to the detected temperature.