The present invention relates to an improved optical device for optical mouse, being installed inside the case body of the optical mouse, comprising: a LED, a lens mount, a sensor and a digital signal processor. That is, instead of using the LED, the LED assembly clip, the positioning holes and the lens mount, which are separated from and independent to one another, for fixing the LED inside the optical mouse, the present invention uses a combined device featuring fool-proof, and positioning-insetting-all-in-one concept can place the LED precisely at the first lens of the lens mount and is capable of installing the LED accurately onto the lens mount and the same time rapidly and effectively adjusting and fixing the relative angle and position as seen in FIG. 6 between the lens mount and the LED. Moreover, the combined device is easy to positioned such that the forgoing shortcomings of high manufacturing cost and difficult to mass-produced are avoided.