This paper proposes a novel power smoothing control method using a spline function for a photovoltaic generation system. The spline function is sequentially created using the preceding average values of PV power. The command value of power smoothing control is determined by extrapolating the obtained spline function, which represents the long-term fluctuation components of PV power. The smoothing effect is evaluated by calculating the power in the targeted frequency band of the power flowing backward to the power grid. The proposed method is verified with 153 days of data measured in the PV system rated 1.75 MW. As a result, the smoothing effect was 51.8% with the proposed method in the case that the parameter of section time was 300 s. The rated capacity of energy storage system was 106 kWh. Then, it was indicated that the smoothing effect was increased and the short-term fluctuation of power flowing back to the power grid was reduced by setting the section time longer. Finally, the proposed method was compared with the power smoothing control using simple MA.