When conveying an envelope, a posture of the envelope is corrected by hitting a closed portion of the envelope, which is opposite side of an opened portion of the envelope, against a nip portion between upper and lower rollers of the pre-registration rollers and the envelope is then conveyed with the envelope being nipped by the upper and lower rollers of the pre-registration rollers. The upper and lower rollers of the pre-registration rollers nip the envelope to remove air from the envelope. When setting an envelope mode to control a conveying speed of the envelope based on a coverage rate of an image to be formed on the envelope, the conveying speed of the envelope is controlled by adjusting a rotation speed of the registration rollers based on the coverage rate of the set envelope mode.