A production plan is created so as to minimize risk of demand variations. Forecasted values of amounts of requests are entered for each different item of products, markets, dates, past accuracy of demand forecasts, and order achievements at the time when the plan is created. Scenario information is created in which assumable amounts of request for each different item of the products, markets, and dates and probabilities at which the amounts of requests agree with actual amounts of requests are defined. Management indexes are found from the amount of stockout and amount of stock calculated in each different scenario, based on the scenario information, target values of the management indexes, and information on strongpoints. An amount of production that maximizes the achievement ratios of the management ratios to their targets within the ranges of supplied materials and within the range of the production capacity is calculated by an optimization algorithm.