To always obtain stable primary transferring ability even when a variation in resistance value of an intermediate transfer belt or a variation in a resistance value of a primary transfer roll is generated.

In the device, each primary transfer roll 8y, 8m, 8c and 8bk is controlled to a constant current at 5 μA during ATCV control 1, voltage generated in each primary transfer roll at the time is detected for 1 rotation of the intermediate transfer belt 5, unevenness in variation of voltage (unevenness in resistance of the intermediate transfer belt and the primary transfer roll) at the time is detected and averaged data is stored in a CPU 15. After that, the primary transfer roll 8 is controlled to the constant voltage (ATV control 2) that is the voltage stored in the CPU 15 when the image is formed.