A variable-speed driving device particularly adapted for an automotive internal combustion engine and associated AC generator. The device includes a driver shaft, such as the crankshaft, a constant-speed-ratio driven shaft, a variable-speed-ratio driven shaft, such as the AC generator shaft, two driver pulleys of different diameters mounted on the driver shaft, a single constant-speed-ratio driven pulley mounted on the constant-speed-ratio driven shaft, and two speed-changing pulleys mounted on the variable-speed-ratio driven shaft. One driving belt is trained over a drive pulley, single constant-speed-ratio driven pulley, and one pulley and an additional belt is trained around the second drive pulley, the second speed-changing pulley, and, if desired, over another constant-speed-ratio pulley on another shaft. There is a changeover mechanism for selectively coupling the speed-changing pulleys to the variable-speed-ratio driven shaft at the desired speed ratio depending on the speed of the driver shaft.