A power system operation plan creation assistance device and method capable of maintaining or improving system stability or economy includes an optimization target snapshot power flow determination unit that uses one or more pieces of foundational information to determine an optimization target snapshot power flow. The foundational information includes a power generation plan, a total demand prediction, a sales plan, a renewable energy prediction, a work stoppage plan, system data, and a setting value. A system manipulation variable candidate extraction unit uses the one or more pieces of foundational information and the optimization target snapshot power flow to extract a candidate for a variable for a variable manipulating the system. An optimal system configuration calculation unit uses the one or more pieces of foundational information and the system manipulation variable candidate to calculate an optimal system configuration.