The device has gear wheels (3,4) that are arranged on shafts (1,2) respectively for transmission of rotational torque. A ball bearing (7) is arranged in bearing ring (6). A piezoelectric transducer (13) is arranged between bearing ring and ball bearing. A rotational torque determining unit determines the transmitted rotational torque based on output signal from piezoelectric transducer. An independent claim is included for rotational torque measurement method.