A control method of a feeder (10, 20) including: a feeding portion which is configured to store a sheet of paper containing an envelope with a predetermined size and to feed the stored sheet of paper, wherein the envelope contains an opened portion and a closed portion; and a pair of registration rollers (13, 23) containing an upper roller and a lower roller, each roller (13,23,31,32) having a longitudinal length longer than a width of the fed sheet of paper, wherein the registration rollers (13,23) are configured to correct a posture of the sheet of paper fed from the feeding portion to convey the sheet of paper toward a downstream side along a sheet-conveying direction, wherein, when conveying the envelope, the posture of the envelope is corrected by hitting the closed portion of the envelope, which is opposite side of the opened portion of the envelope, against a nip portion between the upper and lower rollers (31,32) of the registration rollers (13,23) and the envelope is then conveyed with the envelope being nipped by the upper and lower rollers (31,32) of the registration rollers (13,23), characterised in that the method comprises: a setting step of 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; and a control step of controlling the conveying speed of the envelope by adjusting a rotation speed of the registration rollers (13,23) based on the coverage rate of the envelope mode set in the setting step. a feeding portion which is configured to store a sheet of paper containing an envelope with a predetermined size and to feed the stored sheet of paper, wherein the envelope contains an opened portion and a closed portion; and a pair of registration rollers (13, 23) containing an upper roller and a lower roller, each roller (13,23,31,32) having a longitudinal length longer than a width of the fed sheet of paper, wherein the registration rollers (13,23) are configured to correct a posture of the sheet of paper fed from the feeding portion to convey the sheet of paper toward a downstream side along a sheet-conveying direction, wherein, when conveying the envelope, the posture of the envelope is corrected by hitting the closed portion of the envelope, which is opposite side of the opened portion of the envelope, against a nip portion between the upper and lower rollers (31,32) of the registration rollers (13,23) and the envelope is then conveyed with the envelope being nipped by the upper and lower rollers (31,32) of the registration rollers (13,23), characterised in that the method comprises: a setting step of 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; and a control step of controlling the conveying speed of the envelope by adjusting a rotation speed of the registration rollers (13,23) based on the coverage rate of the envelope mode set in the setting step. a setting step of 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; and a control step of controlling the conveying speed of the envelope by adjusting a rotation speed of the registration rollers (13,23) based on the coverage rate of the envelope mode set in the setting step.