To reduce the impact force by rock falling, sand cushion with 90 cm height is commonly used on the roof of rock-shed. The impact force in this case is usually evaluated by the conventional equation which is derived based on Hertz's contact theory. However, according to the analytical results by one-dimensinal and/or axisymmetric wave theory, the transmitted impact force to rigid base is to be twice of the inputed one. In this paper, to get the real impact force caused by rock falling, the large field test on heavy weight falling on sand cushion is performed. The heavy weights of 2 and 3 tf weight are used and they are dropped from a maximum 30m height. Measuring weight acceleration and transmitted stresses on the rigid base, two kind of impact force are estimated which are obtained from weight acceleration and summing up the transmitted stresses. From the results, it has been shows that 1) the maximum transmitted impact force may become the twice of one obtained from dropped weight, 2) the maximum transmitted impact force can be estimated using the conventional equation assuming Lame's constant λ = 1000 tf/m^2.