An object of the present invention is to achieve favorable injection molding with reduced mold clamping force required for molding and with suppression of weld line occurrence without repeating trial and error manually, by prompt calculation of adequate production parameters, when conducting injection molding of resin products. When the injection molding is conducted using a mold having a plurality of resin inflow conduits N, R, G1, G2, and G3 to the cavity CV, the combination of a numerical analysis method for calculating the injection molding process and a computer-aided optimization method, derives the production parameters which determine time-sequentially the inflow of resin material through resin inflow conduits.