A sampling frequency control method includes a difference calculating step ST1 of calculating the difference between sampled values obtained through sampling of an AC quantity of electricity of a power system at time points that are separated by a prescribed time period of the AC quantity of electricity; an effective value calculating step ST2 of calculating an effective value of the AC quantity of electricity and an effective value of the difference; a ratio calculating step ST3 of calculating the ratio K of the effective value of the difference to the effective value of the AC quantity of electricity; and a control step of controlling the sampling frequency by controlling the sampling period on the basis of the calculated ratio K so that it follows the frequency of the AC quantity of electricity.