PURPOSE: To constantly and steadily keep recording quality by detecting the change of environment, for example the change of humidity which causes change in the resistance of a recording sheet and controlling a transferring bias voltage by means of the detection.

CONSTITUTION: A recording sheet 8 is fed and is traveled in an F direction in velocity equal to the peripheral velocity of a photosensitive body drum 1. At the same time, a humidity sensor 12 and a bias voltage control circuit 13 are operated, and the transferring bias voltage responding to humidity is impressed on a transfer roller 9. Then, light image irradiation is performed by a scanner 4 in response to a recording signal from a recording circuit, and an electrostatic latent image 5 is formed, and the visible image of toner is obtained by a developing device 6. When the leading edge of the recording sheet arrives at a transfer point G, the roller 9 is brought into press-contact with the drum 1, and the visible image of toner 7 on the drum 1 is transferred to the recording sheet.