A method includes: executing judgment processing for judging whether a site to carry out a first task assigned to a first robot and a site to carry out a second task assigned to a second robot are likely to overlap each other; executing comparison processing for comparing first end time with second end time when the sites to carry out the first task and the second task are likely to overlap each other; and executing determination processing for selecting the first task order when the first end time is earlier than the second end time, and selecting the second task order when the second end time is earlier than the first end time.