To provide a method for adjusting an optical scanning device, capable of completing the adjustment in a short time without operating an image forming apparatus itself, and preventing vibration of an optical member in conformation to various vibration frequencies to obtain a high-quality image.

In the method for adjusting the optical scanning device 1, a support part 30 of a reflecting mirror 22 that is the optical member includes a support member 31 and a plate spring 32 which supports the reflecting mirror 22 by pressing the reflecting mirror 22 to the support member 31, vibration generated by at least one excitation device 40 disposed in a housing 2 is measured by a vibration measuring device 41 disposed at the position of the reflecting mirror 2, and a support condition of the support part 30 to the reflecting mirror 22 is changed based on the vibration condition. According to this, vibration to be propagated to the reflecting mirror 22 is generated without mounting and operating the optical scanning device 1 on the image forming device, whereby the unique frequency of a vibration system composed of the reflecting mirror 22 and the support part 30 can be recognized, and resonance can be also avoided.