In a mono component toner developing device, a developing roller supporting and conveying the mono component toner is arranged in contact with a photoreceptor supporting a static latent image. A supplying roller is arranged opposing the developing roller with a predetermined contact depth. The supplying roller has a pair of driven gears fixed at both ends of the rotational shaft thereof for being rotated so as to be meshed with a pair of driving gears rotated by a driving motor. The rotational force of the driving gears, acting at a coupling position, is used to deflect the supplying roller toward the developing roller side, whereby the supplying roller is pressed in approximately uniform contact with the developing roller across its full length along the shaft.