We measured differential pressure distribution on the wing surface of an insect-modeled ornithopter using MEMS sensors during free flight. Three MEMS differential pressure sensors were attached to the wing surface along the wingspan. The distribution was measured when the ornithopter flied freely. The point of the local maximum differential pressures (46 Pa~-42 Pa, 33 Pa~-41 Pa, 9 Pa~-11 Pa)lagged 1.1 ms from the wing root to the wing tip. This result indicates that 3D vortex will be exit on the wing surface, and it affects the flight.