A supply and demand adjustment monitoring device of a power grid includes an output actual power value database that stores an output actual power value of at least one generator, an output power command value database that stores an output power command value issued to the generator, a planned adjustment capacity value database that stores a planned adjustment capacity value for the generator, and an adjustment capacity evaluation unit that evaluates an adjustment capacity based on the output actual power value, the output power command value, and the planned adjustment capacity value.