To provide a means for easily, simply, quickly and accurately calculating instantaneous reactive power and a reactive power effective value and provide a method for measuring them.

Instantaneous values of a voltage V(t) are sampled from time t-(1+(1/4))T to time t-(1/4)T at intervals of time Δt=(1/12)T. Instantaneous values of a current I(t) are sampled from time t-1T to time t at intervals of time Δt. The reactive power effective value Qe(t) is obtained from the instantaneous value data by using formula (52).