A fixing device that performs a fixing operation of a high definition mode toner image on a recording medium, comprising: a fixing rotary body heated by a heater and comprising a fixing belt; and a pressing rotary body separatably pressed against the fixing rotary body to form a nip therebetween through which the recording medium bearing the toner image passes, a temperature detector disposed opposite the fixing rotary body and in contact with the fixing belt to detect a temperature of the fixing rotary body; characterized by a temperature controller connected to the temperature detector, the heater, and the pressing rotary body to control the temperature of the fixing rotary body based on the temperature of the fixing rotary body detected by the temperature detector so as to heat the fixing rotary body to a plurality of preset target temperatures including a first target standby temperature, a target fixing temperature, a target idle temperature, and a second target standby temperature (T3, T4), wherein, the target fixing temperature is lower than the first target standby temperature, wherein the temperature controller separates the pressing rotary body from the fixing rotary body to idle the fixing rotary body for a predetermined idle time period until a temperature of the fixing rotary body decreases to the target idle temperature before the fixing operation at the target fixing temperature starts, and wherein, after the fixing operation, the temperature controller controls the heater to change the temperature of the fixing rotary body to the second target standby temperature (T3, T4), which is lower than the first target standby temperature; and increasing the temperature of the fixing rotary body to the first target standby temperature if the fixing device does not accept a second fixing job within a predetermined time period. a fixing rotary body heated by a heater and comprising a fixing belt; and a pressing rotary body separatably pressed against the fixing rotary body to form a nip therebetween through which the recording medium bearing the toner image passes, a temperature detector disposed opposite the fixing rotary body and in contact with the fixing belt to detect a temperature of the fixing rotary body; characterized by a temperature controller connected to the temperature detector, the heater, and the pressing rotary body to control the temperature of the fixing rotary body based on the temperature of the fixing rotary body detected by the temperature detector so as to heat the fixing rotary body to a plurality of preset target temperatures including a first target standby temperature, a target fixing temperature, a target idle temperature, and a second target standby temperature (T3, T4), wherein, the target fixing temperature is lower than the first target standby temperature, wherein the temperature controller separates the pressing rotary body from the fixing rotary body to idle the fixing rotary body for a predetermined idle time period until a temperature of the fixing rotary body decreases to the target idle temperature before the fixing operation at the target fixing temperature starts, and wherein, after the fixing operation, the temperature controller controls the heater to change the temperature of the fixing rotary body to the second target standby temperature (T3, T4), which is lower than the first target standby temperature; and increasing the temperature of the fixing rotary body to the first target standby temperature if the fixing device does not accept a second fixing job within a predetermined time period.