An image forming method begins with a first image transfer step for thermally transferring a first toner image from a first image carrier to a second image carrier contacting it. Subsequently, in a second image transfer step, the first toner image carried on the second image carrier and a second toner image newly formed on the first image carrier are thermally transferred to opposite sides of a recording medium substantially at the same time. Higher image transfer temperature is assigned to the second image transfer step than to the first image transfer step. An impurity collecting device is associated with a pair of registration rollers which stop the recording medium for a moment to thereby synchronize the recording medium to the toner image.