Inertia force acting on a horizontal circular cylinder in waves significantly decreases with in-creasing Kc number at low Kc numbers. The rapid reduction of inertia force is caused by circulating flow around the cylinder. The strength of the circulation around a horizontal cylinder in regular waves is approximately estimated by using a simple vortex shedding model. From this model, it is deduced that the inertia coefficient is given by C_m=2-(2/π^2)Kc^2. The results by this simple formula are in good agreement with experimental ones at Kc less than 2.5. Wave forces acting on a horizontal circular cylinder in irregular waves are also estimated by using the simple vortex shed-ding model.