A torque sensor for a belt-driven conical-pulley transmission. A first ramp surface is rigidly connected to a shaft of a conical disk pair, and a second ramp surface is rigidly connected to a sensing piston surrounding the shaft and axially and rotatably movable relative to the shaft. Balls are positioned between and in contact with the ramp surfaces and with a guide surface rigidly connected with an axially movable disk. The guide surface is so configured that the contact points of a particular ball with the ramp surfaces, the contact point of that ball with the guide surface, and the center point of that ball lie in a radially oriented plane.