A gear-type key switch of a keyboard device of the present invention includes a key top, a holder member, four gears, a spring member, a film circuit board and a supporting plate. The spring member and the holder member are located above the supporting plate and below the key top. The film circuit board is located between the supporting plate and the spring member. The holder member forms four pairs of shafts and the four gears are respectively rotatably assembled to the four pairs of shafts. The key top forms at least four rack supporting members on a bottom thereof. The rack supporting members are respectively parallel to the four gears and engage with the four gears to provide the upward and downward movement of the key top. The gear-type key switch of a keyboard device is simple in structure, with low friction force and decreased height. In addition, the gears can be assembled in mass via special assistant tools, and the keyboard device is also featured in perfect pressing handle, super-thin and easy assembly.