An image forming apparatus includes: an image carrier; a developing housing that accommodates a developer for developing an electrostatic latent image into a developer image; a developer carrier, which carries the developer in the developing housing, and which supplies the developer to the image carrier; a developing bias supply part that supplies a developing bias to the developer carrier; a remaining quantity calculation part that calculates a remaining quantity of the developer in the developing housing; a deterioration grade storage part that stores a relationship between the remaining quantity of the developer in the developing housing and a deterioration degree of the developer caused by the developing operation; and a developing bias setting part that sets a value of the developing bias in accordance with the remaining quantity calculated by the remaining quantity calculation part and the relationship stored in the deterioration grade storage part.