Plural combined processing machines perform various processes to workpieces W. The combined processing machines are arranged in a processing line according to a volume of products of the workpieces, operating time of the combined processing machines and a tact time to complete processing one of the workpieces. Each combined processing machine is determined to share one or more processes according to the volume of the products, the operating time and the tact time. The workpieces are transferred among the combined processing machines to be processed.