A power measurement apparatus comprising: a voltage detector (3, 30) that detects in a contactless manner an AC voltage of a conductive path to which power is supplied from an AC power source whose magnitude of AC voltage is regulated to be a predetermined value and that outputs first data signal regarding a voltage waveform of the AC voltage of the conductive path; a current detector (4, 10) that detects in a contactless manner an AC current flowing through the conductive path and that outputs second data signal regarding a current waveform of the AC current of the conductive path; and a power calculator that receives the first data signal and the second data signal and that calculates active power of the conductive path from a product of a second instantaneous voltage and an instantaneous current of the current waveform indicated by the second data signal, the second instantaneous voltage being generated by converting a first instantaneous voltage of the voltage waveform indicated by the first data signal on the basis of the predetermined value, wherein the voltage detector includes a probe (5, 5r, 5s, 5t), an input part, a differential amplifier, and a regulator, the probe includes an electrode arranged so as not to come into contact with the conductive path, and generates an impedance including a capacitive component between the conductive path and the electrode, the input part is connected to the probe and produces an input signal corresponding to a waveform of the AC voltage of the conductive path on the basis of a potential of the electrode, the differential amplifier amplifies the input signal produced by the input part and outputs an output signal, the output signal being the first data signal, and the regulator regulates at least one of a gain of the differential amplifier and a magnitude of the input signal produced by the input part on the basis of a magnitude of the output signal of the differential amplifier, and keeps the magnitude of the output signal of the differential amplifier within a predetermined range, wherein the regulator is configured such that a magnitude of the output signal of the differential amplifier obtained when the gain is regulated to a minimum is allowed to be smaller than one half of a magnitude of the output signal of the differential amplifier obtained when the gain is regulated to a maximum. a voltage detector (3, 30) that detects in a contactless manner an AC voltage of a conductive path to which power is supplied from an AC power source whose magnitude of AC voltage is regulated to be a predetermined value and that outputs first data signal regarding a voltage waveform of the AC voltage of the conductive path; a current detector (4, 10) that detects in a contactless manner an AC current flowing through the conductive path and that outputs second data signal regarding a current waveform of the AC current of the conductive path; and a power calculator that receives the first data signal and the second data signal and that calculates active power of the conductive path from a product of a second instantaneous voltage and an instantaneous current of the current waveform indicated by the second data signal, the second instantaneous voltage being generated by converting a first instantaneous voltage of the voltage waveform indicated by the first data signal on the basis of the predetermined value, wherein the voltage detector includes a probe (5, 5r, 5s, 5t), an input part, a differential amplifier, and a regulator, the probe includes an electrode arranged so as not to come into contact with the conductive path, and generates an impedance including a capacitive component between the conductive path and the electrode, the input part is connected to the probe and produces an input signal corresponding to a waveform of the AC voltage of the conductive path on the basis of a potential of the electrode, the differential amplifier amplifies the input signal produced by the input part and outputs an output signal, the output signal being the first data signal, and the regulator regulates at least one of a gain of the differential amplifier and a magnitude of the input signal produced by the input part on the basis of a magnitude of the output signal of the differential amplifier, and keeps the magnitude of the output signal of the differential amplifier within a predetermined range, wherein the regulator is configured such that a magnitude of the output signal of the differential amplifier obtained when the gain is regulated to a minimum is allowed to be smaller than one half of a magnitude of the output signal of the differential amplifier obtained when the gain is regulated to a maximum.