We present a numerical method to calculate the magnetic shape anisotropy in rough thin films. The magnetic shape anisotropy is calculated LS considering the magnetostatic energy between the surface magnetization induced at rough surfaces of ferromagnetic thin films. Using the present method, we quantitatively evaluated the sur face shape anisotropy from the reduction of the shape anisotropy energy with respect to the surface roughness. The shape anisotropy of real Ni/Pt multilayer films was revealed to be reduced by more than 10 % due to the surface shape of the roughness.