This paper presents the experimental results of the fluid forces acting on a circular cylinder placed in the oscillating flow at Keulegan-Carpenter number up to 38. The drag and inertia coefficients are determined through the use of Fourier average in applying Morison's formula to measured fluid forces. Further, the applicability of this formula is examined. Fluid forces (in-line and transverse forces) were measured simultaneously. The visualization with aluminum dust shows that vortex shedding from the cylinder is devided into some patterns. Strouhal number in some range of KC number became smaller than that of seady flow because the vortex shedding is restricted by a return flow.