The WIG (Wing-In-Ground effect) is an unconventional aircraft and /or a ship which is operated in a seaway utilizing the high lift-to-drag ratio. When the WIG flies over the waves, unsteady aerodynamic forces induced by the waves act on the airframe and cause the unsteady change of the pitching moment with the movement of the center of pressure. They have a possibility to cause the instability of the motion of the airframe. Therefore, the characteristics of unsteady aerodynamic forces become an important topic related to WIG's flight stability. This paper takes up the WIG flying with heave motion in the ground effect and makes clear the property of unsteady aerodynamic forces and moment. The Time-domain boundary element method is employed for analyzing the problem and the validation of the obtained results are also carried out by comparing with experimental results. In the experiment, not only the measurement of unsteady forces and moment but also the measurement of the unsteady pressure field over the ground are attempted by applying the technique to measure the unsteady waves of ships. The later one is the first trial in this research field and obtained experimental data itself is valuable from the academic point of view.