To prevent the rising of temp., without increasing the cost of a device by executing control so as to make the air quantity of a heat discharge fan larger, when the size of a recording paper is smaller than a previously set prescribed size.

A size detecting part 201 detects the size of the recording paper and informs a CPU 202. This CPU 202 compares the size of the recording paper detected by the size detecting part 201 with a reference size capable of making paper passage by a thermal fixing roller. When it is judged that the detected size of the recording paper is smaller, the CPU 202 controls the heat discharge fan 101, to make its air quantity larger. At this time, a special member is not required for discharging heat, so that the heat of a paper nonpassing part can be discharged without increasing the cost of the fixing device. Since the fan 101 discharges the heat of the paper nonpassing part of the thermal fixing roller, freedom in terms of the critical temp. of the thermal fixing roller can be obtained as well and the number of consecutive paper passing times can be increased.