A method of controlling frequency of a power system by a frequency control system includes: obtaining a frequency control command to keep frequency fluctuation of the power system within a predetermined range; obtaining information on electric power to be received or supplied by m second apparatuses, and determining n first command values for controlling electric power to be received or supplied by n first apparatuses, based on the frequency control command obtained in the obtaining of a frequency control command and the obtained information on electric power to be received or supplied by the m second apparatuses; and transmitting each of the n first command values determined in the determining to a corresponding one of the first apparatuses.