We propose a method to investigate mechanisms of insect flight using motor-driven ornithopter with acceleration sensors. The ornithopter was designed to have 110 mm semi-wingspan and weigh 6 g. The Reynolds number of the ornithopter was the same order as that of insects (Re ~ 10⁴). We measured accelerations of the ornithopter with 300 Hz for 0.4 sec using the sensors fixed on the ornithopter. The ornithopter weight including the sensor circuit was 6.1 g, which was light enough to attach on the ornithopter. Sensor outputs were once stored in EEPROM and retrieved later. The measured angular velocity and acceleration during free flight were 600 degree/s peak-to-peak and 30 m/s² peak-to-peak, respectively.