We investigated flapping-flight mechanisms of insects using a motor-driven ornithopter with FM-transmitter. Wing length and total weight of the ornithopter were respectively 150 mm and 7.0 g. Weight of the FM-transmitter was 1.3 g. Moment acting on the wing base of the ornithopter was measured with strain gages attached to the base of the wing frame. Signals from the strain gages were transmitted by the FM-transmitter and received by a radio tuner for measurements in free flights. We compared wave forms of the signals in free flights with those in tethered flights. As a result, the wave forms in free flights were different from those in tethered flights. This result suggested that the aerodynamic forces generated by flapping wings in free flights were different from those in tethered flights.