A power supply apparatus (10; 20) comprising: a converter circuit (12; 22) configured to convert an alternating current output from an AC power supply to a direct current; a battery (15; 25) that is configured to be charged by the direct current output from the converter circuit (12; 22); a first power supply circuit (13; 23) configured to output a first voltage that is within an allowable range of a load voltage of a load circuit (2; 3) from either one of the converter circuit (12; 22) and the battery (15; 25) to the load circuit (2; 3); a second power supply circuit (14; 24) configured to output either one of a second voltage that is within the allowable range and a third voltage that is lower than the first voltage from either one of the converter circuit (12; 22) and the battery (15; 25) to the load circuit (2; 3); and a power supply control unit (11; 21) configured to cause the first power supply circuit (13; 23) to output the first voltage and the second power supply circuit (14; 24) to output the third voltage, when the alternating current is output from the AC power supply, and further configured to cause the second power supply circuit (14; 24) to output the second voltage and the first power supply circuit (13; 23) to stop outputting the first voltage, when the AC power supply is stopped, characterised by wherein both of the first power supply circuit (13; 23) and the second power supply circuit (14; 24) comprise a backflow prevention diode configured to regulate a current direction in the power supply apparatus (10; 20) to conform to a current direction of the load circuit (2; 3), and wherein the second power supply circuit (14; 24) is configured to output a fourth voltage that is lower than the first voltage when the power supply apparatus (10; 20) is in a standby state, the fourth voltage being set such that the upper limit of a fluctuation range of the fourth voltage is smaller than the lower limit of a fluctuation range of the first voltage. a converter circuit (12; 22) configured to convert an alternating current output from an AC power supply to a direct current; a battery (15; 25) that is configured to be charged by the direct current output from the converter circuit (12; 22); a first power supply circuit (13; 23) configured to output a first voltage that is within an allowable range of a load voltage of a load circuit (2; 3) from either one of the converter circuit (12; 22) and the battery (15; 25) to the load circuit (2; 3); a second power supply circuit (14; 24) configured to output either one of a second voltage that is within the allowable range and a third voltage that is lower than the first voltage from either one of the converter circuit (12; 22) and the battery (15; 25) to the load circuit (2; 3); and a power supply control unit (11; 21) configured to cause the first power supply circuit (13; 23) to output the first voltage and the second power supply circuit (14; 24) to output the third voltage, when the alternating current is output from the AC power supply, and further configured to cause the second power supply circuit (14; 24) to output the second voltage and the first power supply circuit (13; 23) to stop outputting the first voltage, when the AC power supply is stopped, characterised by wherein both of the first power supply circuit (13; 23) and the second power supply circuit (14; 24) comprise a backflow prevention diode configured to regulate a current direction in the power supply apparatus (10; 20) to conform to a current direction of the load circuit (2; 3), and wherein the second power supply circuit (14; 24) is configured to output a fourth voltage that is lower than the first voltage when the power supply apparatus (10; 20) is in a standby state, the fourth voltage being set such that the upper limit of a fluctuation range of the fourth voltage is smaller than the lower limit of a fluctuation range of the first voltage.