An optical scanning device for scanning a first circumferential surface as a surface to be scanned of a first photoconductive drum rotatable about an axis and a second circumferential surface as a surface to be scanned of a second photoconductive drum rotatable about an axis with light beams, comprising: a first light source for emitting a light beam; a second light source for emitting a light beam; a polygon mirror including a plurality of mirror surfaces (R1∼R6) and configured to scan the first circumferential surface in a first direction along a main scanning direction by deflecting a first light beam emitted from the first light source and, on the other hand, scan the second circumferential surface in a second direction opposite to the first direction by deflecting a second light beam emitted from the second light source while rotating about an axis; a first scanning lens (32,33) for imaging the first light beam on the first circumferential surface and a second scanning lens (42,43) for imaging the second light beam on the second circumferential surface; a first reflecting mirror for reflecting the first light beam so that the first light beam propagates toward the first circumferential surface, the first reflecting mirror being arranged on optical path between the polygon mirror and the first circumferential surface, and a second reflecting mirror for reflecting the second light beam so that the second light beam propagates toward the second circumferential surface, the second reflecting mirror being arranged on optical path between the polygon mirror and the second circumferential surface; and characterised by an adjusting mechanism (49,53) for adjusting the posture of at least the first reflecting mirror, wherein the adjusting mechanism: adjusts the posture of the first reflecting mirror using a predetermined first adjustment amount so that a first scanning line drawn by the first light beam to scan the first circumferential surface and a second scanning line drawn by the second light beam to scan the second circumferential surface are parallel when a rotation speed of the first and second photoconductive drums (2a,2b) is a predetermined first speed, and adjusts the posture of the first reflecting mirror using a predetermined second adjustment amount different from the first adjustment amount and capable of making the first and second scanning lines (SL1,SL2) parallel at a second speed different from the first speed when the rotation speed of the first and second photoconductive drums (2a,2b) is the second speed. a first light source for emitting a light beam; a second light source for emitting a light beam; a polygon mirror including a plurality of mirror surfaces (R1∼R6) and configured to scan the first circumferential surface in a first direction along a main scanning direction by deflecting a first light beam emitted from the first light source and, on the other hand, scan the second circumferential surface in a second direction opposite to the first direction by deflecting a second light beam emitted from the second light source while rotating about an axis; a first scanning lens (32,33) for imaging the first light beam on the first circumferential surface and a second scanning lens (42,43) for imaging the second light beam on the second circumferential surface; a first reflecting mirror for reflecting the first light beam so that the first light beam propagates toward the first circumferential surface, the first reflecting mirror being arranged on optical path between the polygon mirror and the first circumferential surface, and a second reflecting mirror for reflecting the second light beam so that the second light beam propagates toward the second circumferential surface, the second reflecting mirror being arranged on optical path between the polygon mirror and the second circumferential surface; and characterised by an adjusting mechanism (49,53) for adjusting the posture of at least the first reflecting mirror, wherein the adjusting mechanism: adjusts the posture of the first reflecting mirror using a predetermined first adjustment amount so that a first scanning line drawn by the first light beam to scan the first circumferential surface and a second scanning line drawn by the second light beam to scan the second circumferential surface are parallel when a rotation speed of the first and second photoconductive drums (2a,2b) is a predetermined first speed, and adjusts the posture of the first reflecting mirror using a predetermined second adjustment amount different from the first adjustment amount and capable of making the first and second scanning lines (SL1,SL2) parallel at a second speed different from the first speed when the rotation speed of the first and second photoconductive drums (2a,2b) is the second speed. adjusts the posture of the first reflecting mirror using a predetermined first adjustment amount so that a first scanning line drawn by the first light beam to scan the first circumferential surface and a second scanning line drawn by the second light beam to scan the second circumferential surface are parallel when a rotation speed of the first and second photoconductive drums (2a,2b) is a predetermined first speed, and adjusts the posture of the first reflecting mirror using a predetermined second adjustment amount different from the first adjustment amount and capable of making the first and second scanning lines (SL1,SL2) parallel at a second speed different from the first speed when the rotation speed of the first and second photoconductive drums (2a,2b) is the second speed.