An optical scanning device includes first and second light sources, a polygon mirror, first and second scanning lenses, first and second reflecting mirrors, and an adjusting mechanism for adjusting the posture of the first reflecting mirror. 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 a first light beam to scan a first circumferential surface and a second scanning line drawn by a second light beam to scan a second circumferential surface are parallel when a drum rotation speed 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 parallel at a second speed different from the first speed when the drum rotation speed is the second speed.