A variable speed control system includes a multiple-speed transmission which changes a speed of an engine output in multiple stages based on a change in meshing state of a dog clutch which is caused in association with an operation of a change pedal connected mechanically, a pedal load detection device which detects an operation load of the change pedal, and a control unit which suppresses or shuts off an engine output to the multiple-speed transmission and permits a speed change action of the multiple-speed transmission by the change pedal when a detected pedal load exceeds a pedal load threshold. The variable speed control system further includes an engine revolution speed detection sensor which detects an engine revolution speed. The control unit changes the pedal load threshold according to the engine revolution speed detected. Such variable speed control system can realize a stable speed change action irrespective of running conditions.