While the price of electricity in Japan is increasing continuously, the plastic industry consumes considerable electric power for injection molding. The cost of electricity must be reduced to maintain product prices. Some strategies have been proposed to reduce the cost of electricity at the production stage, one of which is to reduce the contract electric energy amount with an electric company while increasing the facility operation rate and reducing excess power. This study proposes a method to minimize the electricity cost of injection molding lines by using the simulation for injection molding lines with an approximation solution. The simulated annealing method was used as the approximation solution. Based on the proposed method, numerous case studies were performed, and the results were analyzed.