An insect can perform flight maneuvers such as hovering and snap turns. There have been many methods such as digital particle image velocimetry (DPIV) or kinematic analysis using robotic wings modeled on insects to investigate the aerodynamic force. The aerodynamic force is derived from the differential pressure between the upper and lower surfaces of its wings. The actual aerodynamic force on real insect wings has never been measured directly during free flight. Here we show direct measurement of differential pressure acting on the wing surface of a real insect during free flight. A micro differential pressure sensor was fabricated by utilizing micro electro mechanical systems (MEMS) and fixed to a butterfly (Papilio protenor) wing. The size and weight of the sensor chip are 1.0 mm x 1.0 mm x 0.3 mm and 0.7 mg, respectively. The differential pressures were measured during takeoff. From the measured differential pressure distribution, aerodynamic force was calculated. The maximum aerodynamic force generated on the wings was 3 times larger than the gravity acting on the body.