Recently, a user uses a right and left functional keys of electronic products, such as a computer mouse, a rod, a keyboard, etc.,  to browse document, search homepage, and enlarge and narrow image.  Also, the user can use a wheel except the right and left functional keys, wherein the wheel is disposed between the right and left functional keys.  A signal is outputted during rotation of the wheel as follows:
Recently, most wheels of the computer mouse is designed and shown in FIG.  1.  The computer mouse includes a fixing seat supported the wheel and an axle passed through and disposed in the center of the wheel, wherein the axle is extended to an optical grating switch.  During rotation of the wheel, the axle synchronously drives an optical grating sheet which is in the optical grating switch for generating and then outputting different signals of command.  But, the wheel is provided with a pressing type switch for exiting the preset functional window of the software and then processing another command when the user presses the pressing type switch, except the wheel outputs the signal of the command during rotation of the wheel.  The axle of the wheel and the optical grating switch are mounted together, and therefore the pressing distance of the wheel is requested to consider in accordance with the design of the optical grating switch.  This increase the difficulty of the design, and also decrease the lifetime of the optical grating switch because the wheel is frequently pressed.  In addition, the manufacture cost of the optical grating switch is high, and the design of the optical grating switch is more complex as the optical grating switch is applied to a computer mouse with smaller size. 
In order to achieve tendency towards the minimization of an electronic product, Taiwan Patent No.  365,419 (i.e. U.S. Pat.  No.  6,188,389) discloses a third axis input device for a computer mouse including a fixed primary seat, a fixed secondary seat, a wheel, an annular ring-like grating member, a suspending member, and an idle roller.  The grating member is hidden, i.e. the grating member is disposed in the wheel for decreasing conventional installation space.  However, the wheel in prior art still has a problem of a high cost of the optical grating switch.  Furthermore, the size of the wheel is smaller because of the computer mouse with smaller size, and therefore it is more difficult to manufacture the optical grating switch disposed in the wheel. 
Accordingly, there exists a need for a scroll wheel device with a thin film type command switch to solve the above-mentioned problems and disadvantages.