To enhance the accuracy of detecting the thickness of recording material by detecting the thickness of the recording material in accordance with a signal from a detection means and changing some condition when an image is formed.

When the leading edge of the recording material 13 reaches a pair of rollers 1 and 2, the recording material 13 is held and carried by a pair of rollers 1 and 2. Since the carrying roller 1 and the pressure roller 2 are meshed each other through gears 3 and 4, periodic waveform is outputted from a sensor 8. From the point of time when the recording material 13 reaches a sensor 10 on the upstream side of a pair of rollers 1 and 2, a pair of rollers 1 and 2 starts rotation. After specified time elapses from the point of time when the rotation is started, data is stored in a memory every fixed time. At the point of time when the number of the data attains a specified number previously stored, the data is added and divided by the specified number, so that an average value is calculated and stored in the memory. Furthermore, the stored value is subtracted. Then, it is compared with a voltage value in the case the recording material having the specified thickness is held by a pair of rollers 1 and 2, which is stored in the memory, so as to discriminate the thickness of the recording material 13 and an image is formed on the optimum image forming condition.