Counter-Meshing Gears(CMG) mechanism is a complex discrimination mechanism which can be used in micro/meso combination locks for high-consequence system surety. Discrimination function of the CMG mechanism is defined by its two composite gears, which are stacked by several coding gears separately. Under the precondition to satisfy the same discrimination functions, the minimized composite gear level is propitious to the miniaturization, reliability and manufacturability of the mechanical combination lock with a CMG mechanism. The minimum composite gear level is one of the most important fundamental problems of the CMG mechanism, whereas it has not been reported. Most needed basic concepts for the proving process are introduced firstly, including discrimination principle of the CMG mechanism, definition of the CMG mechanism coding problem, and delaminating criterion for the CMG mechanism’s optimized coding method. A conclusion that the minimum composite gear level of the typeⅠ CMG mechanism is 3 is proven. And principle of the previously reported 3 colors circular alternant coloring method is also explained. There is a degenerated instance of the typeⅠCMG mechanism when the unlock symbol sequence has only one switch from symbol A to B, or vice versa. Because the degenerated instance is not practical, it can be ignored in engineering applications.