A fixing temperature control device according to the present invention is applied to a copying machine. A control section has a warm-up control mode and a normal control mode. In the warm-up control mode, energization to a heater is started in response to the start of copying, while being stopped at the time point where a temperature detected by a heat roller temperature detecting sensor reaches a first off temperature. In the normal control mode, on-off control of the energization to the heater is carried out utilizing a predetermined control temperature as a target subsequently to the termination of the warm-up control mode. The predetermined control temperature is corrected by adding a prescribed correction amount to a reference control temperature. It is preferable that the required correction amount includes i) an initial correction amount added in the early stages of processing in the normal control mode, ii) a correction amount at the time of passing paper sheets added when paper sheets pass between a heat roller and a pressure roller in a fixing device, and iii) an environmental correction amount added depending on an environmental temperature.