We focus on Japanese electricity exchange market. Currently, the 9 electric power companies in Japan carry the dominant influence as a seller side in JEPX transaction substantially. Therefore the aggregated marginal generation costs of the 9 electric power companies become major factors in the spot pricing mechanism in JEPX. Based on these facts, the fluctuation mechanism of electricity spot prices in JEPX depends on the supply side to a large extent. This paper addresses the challenge of building spot prices modeling for JEPX using a modified hybrid price model which can take not only cost-based issue but also market-based issue into consideration. The price model is decomposed into expected value part and indeterminate part. In order to represent the expected value part, we apply the Hill function, which can well fit the feature of 9 electric power companies’ aggregated marginal cost. We also apply the time series model to represent the indeterminate part. We forecast the spot prices using the model for verification of the model, and validate its good forecasting performance. Our model is suitable for the electricity exchange market in Japan.