A station-building power-supply device comprising: a ripple detection unit that is configured to detect a ripple included in an overhead line voltage by analyzing frequency components of the overhead line voltage and to output information on the detected ripple, together with the overhead line voltage value, VrmsO, at the time of the ripple detection; a ripple determination unit that is configured to compare the information on the ripple with a ripple threshold value for determining whether a ripple is included in the overhead line voltage, and to output, when it is determined that ripple is not included in the overhead line voltage, the overhead line voltage value; a no-load voltage estimation unit that is configured to provide an actual estimate Vn of a varying no-load voltage value, by using the equation Vn = V'rms0 × 1.35/√2, wherein V'rms0 is the minimum overhead line voltage that is output by the ripple determination unit, and Vn is an estimate of the overhead line voltage in a no-load state; and a calculation unit that is configured to calculate a regeneration determining voltage value for determining whether an electric vehicle connected to the overhead line performs regenerative operation, and to output the regeneration determining voltage value to a main circuit section that utilizes regenerative energy generated by the regenerative operation, wherein in calculating the regeneration determining voltage value, the calculation unit calculates the regeneration determining voltage value to be higher than the estminated no-load voltage value Vn by a predetermined value, wherein the main circuit section comprises a comparison unit configured to compare the overhead line voltage value input from a voltage sensor with the regeneration determining voltage value input from the calculation unit, and configured to control a power conversion unit so that power of the overhead line is converted and output to the station building when the overhead line voltage value is greater than the regeneration determining voltage value. a ripple detection unit that is configured to detect a ripple included in an overhead line voltage by analyzing frequency components of the overhead line voltage and to output information on the detected ripple, together with the overhead line voltage value, VrmsO, at the time of the ripple detection; a ripple determination unit that is configured to compare the information on the ripple with a ripple threshold value for determining whether a ripple is included in the overhead line voltage, and to output, when it is determined that ripple is not included in the overhead line voltage, the overhead line voltage value; a no-load voltage estimation unit that is configured to provide an actual estimate Vn of a varying no-load voltage value, by using the equation Vn = V'rms0 × 1.35/√2, wherein V'rms0 is the minimum overhead line voltage that is output by the ripple determination unit, and Vn is an estimate of the overhead line voltage in a no-load state; and a calculation unit that is configured to calculate a regeneration determining voltage value for determining whether an electric vehicle connected to the overhead line performs regenerative operation, and to output the regeneration determining voltage value to a main circuit section that utilizes regenerative energy generated by the regenerative operation, wherein in calculating the regeneration determining voltage value, the calculation unit calculates the regeneration determining voltage value to be higher than the estminated no-load voltage value Vn by a predetermined value, wherein the main circuit section comprises a comparison unit configured to compare the overhead line voltage value input from a voltage sensor with the regeneration determining voltage value input from the calculation unit, and configured to control a power conversion unit so that power of the overhead line is converted and output to the station building when the overhead line voltage value is greater than the regeneration determining voltage value.