Following the first report on the problem of the wave force reduction at low K _c number region in regular waves, the problem in irregular waves are experimentally investigated in the present paper. Measurements of wave forces acting on circular, square and lowerhull cylinders horizontally submerged in irregular waves show a reduction of inertia forces at the moment of high wave amplitude. An existence of a circulating flow round a cylinder is confirmed by flow visualization as same as in regular waves. The reduction of the inertia forces has a similar dependency on K _c number to that in regular waves if a K _c number are defined in terms of each amplitude of water particle. A prediction method of the wave forces acting on a cylinder in irregular waves is proposed.