An ornithopter with an airframe structure that is lightweight, simple and stable and can generate sufficient lift and thrust. The ornithopter comprises a body, a main wing attached to an upper portion of a front section of the body, and a tail wing attached to a rear section of the body. The ornithopter further comprises a power source and a power transmission mechanism installed within a housing of the body. The main wing includes a wing frame composed of a plurality of frame rods and a skin attached to the wing frame for forming an outline of the main wing. The wing frame of the main wing is supported by a support means exposed to the outside at the upper portion of the front section of the body. The power transmission mechanism includes a gear train for adjusting the rotational motion of the power source at a proper speed and transferring it to the main wing, and connecting rods for converting the rotational motion into a swing motion of the main wing. Flight time of the ornithopter can be prolonged considering its weight and the power source can be easily manipulated.