An arc detector includes a current sensor, a power spectrum conversion unit that converts an output from the current sensor to a power spectrum, a section value obtaining unit that divides an arc measurement section for the power spectrum into a plurality of regions, and obtains, from region values for the plurality of regions excluding a maximum one of the region values, a region value to be a section value of the arc measurement section, and an arc determination unit that determines a presence of an arc by comparing the section value with a threshold.