This paper reports on differential pressure distribution measurement of a free-flying butterfly wing with a MEMS differential pressure sensor. A piezoresistive cantilever was used as a differential pressure sensor. The sensor chip was 1.0 mm 1.0 mm 0.3 mm in size and 0.7 mg in weight, respectively, so that it did not interfere with wing motion. Using a butterfly (Papilio protenor) with a sensor chip attached to the wing surface, the distribution of five points to the spanwise direction at the forewing and one point at the center of the hindwing was measured during takeoff. As a result, it was clarified that the maximum aerodynamic force generated on the wing was 3 times larger than the gravity acting on the body.