Profitability of electric power producers is evaluated in this paper when the same secondary regulation market as Spanish one is introduced to Japan. When the power producers participate in the frequency regulation market, they must have the upward output margin for the frequency control, so that the power sale income is reduced while the revenue from the frequency regulation market is generated. These increased and decreased profits when participating in the frequency regulation market are formulated at first in this paper. Based on the formulation, the profitable situation where the power producers should participate in the frequency regulation market is clarified. Using Spanish market data statistics and actual data of JEPX spot market price, the profit of the power producers in the frequency regulation market is simulated. From those results, it is found that there is an economic merit of participating in the frequency regulation market at a specific JEPX spot market price.