Patent ID: 8213812

Claim:
An image forming apparatus, which supplies a new development agent consisting of a toner and a carrier and discharges a deteriorated development agent to maintain a performance of a development agent in a development device, comprising: a stirring information obtaining section configured to obtain information of a stirring time of the development agent in the development device; a supply information obtaining section configured to obtain information of an amount of the carrier supplied into the development device; and a deterioration degree determining section configured to determine a deterioration degree of the development agent in the development device on the basis of the information obtained by the stirring information obtaining section and the supply information obtaining section, and determine the development agent contained in the development device reaches its change time when a deterioration index value of development agent is greater than a predetermined threshold value, where the deterioration index of development agent=(previous deterioration index of development agent +A)×(1−B)×100, the deterioration index of development agent representing a deterioration degree of a development agent, where A is a rate of deterioration from a previous print JOB end to a current print JOB end and B is a refreshed rate from a previous print JOB end to a current print JOB end.