Up to now, it has universally been thought that provided the input torque of the driving gear of the involute gearing keeps constant, its supports do not vibrate. It has been proved by the researches in this article that the involute gear supports generate inevitably the forced vibration to a certain extent during operation. The author of this paper has made systematical analyses and researches on the changing laws of the actual reactions of the gear supports in operation, and derived the expressions for the actual reactions and a group of equations for calculating the fluctuation rate of the actual reactions. The close mathematical and mechanical relationships, which are described and expressed by the group of equations, reveal clearly that the main implicit factor leading to the forced vibration of the supports does be the sliding friction between teeth. Deeper researches and analyses have further indicated that the important measures for restraining the vibration of the supports are: reducing effectively the sliding friction coefficient between teeth, adjusting reasonably the tooth numbers and the transmission rate, and controlling strictly the center distance increment coefficient.