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This paper reports development of a measuring device built in a camshaft drive chain sprocket for detecting precise torque fluctuation with minimum change on the valve drive system dynamical characteristics. This torque measuring device (TMD) was designed to measure torque with minimum cross-talk with bending force or lateral force. To realize these functions, the disc portion of a chain sprocket that connects the teeth and the camshaft was carved to make thin plate area so that the strain gages placed on the area may have enough sensitivity to torque fluctuation. The signal from the circuit goes through frequency modulation and is transmitted to demodulation circuit via coil antennas. By this arrangement, almost perfect linearity was observed in the relationship of the TMD output voltage and the applied torque. Using a model cylinder head unit of an in-line four cylinder gasoline engine driven by a variable speed electric motor, the TMD performance was evaluated.