PURPOSE: To switch the mode of a developing device to a mode in which the density of a slid image has priority and another mode in which the reproducibility of a low- density narrow line has priority by switching set conditions of the developing device.
CONSTITUTION: By rotating a magnetic roll 6, a developing magnetic pole N 1 to a developing area X is positioned on a center line L connecting the rotating center O 5 of a developing sleeve 5 and the position Y at which the sleeve 5 and an electrostatic latent image carrying body 1 become the nearest to each other and a regulating member 7 is set to the 1st state by facing a magnetic pole S 2 to the member 7. Then, by further rotating the magnetic roll 6, the magnetic pole N 1 is positioned to the upstream side of the position Y in the rotating direction of the sleeve 5 and the member 7 is set to a 2nd state by facing the nearly intermediate section of adjacent magnetic poles S 2 and N 4 . A switching means which switches the above-mentioned two states to each other is provided. Therefore, a mode in which the density of a solid image is secured and another mode in which the reproducibility of a low-density narrow line has priority can be switched easily to each other by only rotating the magnetic roll by prescribed angles and an original can be reproduced appropriately in accordance with the image quality.