For reducing CO_2 emissions and fuel cost reduction, energy-saving modification is necessary even for existing fishing vessels. For this purpose, a fishing vessel with high Froude number, the reduction of wave-making resistance is important. In this study, the author discuss about the constraints in the case of applying the minimum wave-making resistance theory to an existing hull form. In this paper, the author reports on the calculation results, if the Cp curve of an existing hull form were fixed at several points. By the number of constraint is sometimes rippling Cp curve has been computed, it should be noted that the position and number of constraint was found. In addition, if the restraint is too small Cp value yielded a partial Cp curve becomes negative near the bow.