An air conditioner comprising: a plurality of indoor units; and at least one outdoor unit (M, S1, S2), wherein the outdoor unit includes a control unit which controls the operation of the outdoor unit and comprises a PFC circuit that converts AC power provided by a commercial AC power source into DC power and a compressor inverter that drives a compressor motor by converting the DC power into AC power, the PFC circuit and the compressor inverter both being mounted on a first substrate, wherein the control unit further comprises a system controller which controls the operation of the outdoor unit, and characterised in that the system controller is configured to control the PFC circuit not to operate if the frequency or magnitude of the AC power is beyond a predetermined range, if a compressor operating signal is yet to be input, if DC voltage output by the PFC circuit is beyond a predetermined range, or if speed or power of a compressor is lower than a reference value, wherein the control unit further comprises: at least one fan inverter which drives at least one fan motor (71, 72) by converting the DC power into AC power, and a filter which removes noise between the commercial AC power source and the PFC circuit, wherein the at least one fan motor inverter is mounted on a second substrate, the system controller is mounted on a third substrate, and the filter is mounted on a fourth substrate, wherein the fourth substrate is disposed below the third substrate, wherein the system controller controls the compressor inverter, the at least one fan motor inverter, and the PFC circuit. a plurality of indoor units; and at least one outdoor unit (M, S1, S2), wherein the outdoor unit includes a control unit which controls the operation of the outdoor unit and comprises a PFC circuit that converts AC power provided by a commercial AC power source into DC power and a compressor inverter that drives a compressor motor by converting the DC power into AC power, the PFC circuit and the compressor inverter both being mounted on a first substrate, wherein the control unit further comprises a system controller which controls the operation of the outdoor unit, and characterised in that the system controller is configured to control the PFC circuit not to operate if the frequency or magnitude of the AC power is beyond a predetermined range, if a compressor operating signal is yet to be input, if DC voltage output by the PFC circuit is beyond a predetermined range, or if speed or power of a compressor is lower than a reference value, wherein the control unit further comprises: at least one fan inverter which drives at least one fan motor (71, 72) by converting the DC power into AC power, and a filter which removes noise between the commercial AC power source and the PFC circuit, wherein the at least one fan motor inverter is mounted on a second substrate, the system controller is mounted on a third substrate, and the filter is mounted on a fourth substrate, wherein the fourth substrate is disposed below the third substrate, wherein the system controller controls the compressor inverter, the at least one fan motor inverter, and the PFC circuit. at least one fan inverter which drives at least one fan motor (71, 72) by converting the DC power into AC power, and a filter which removes noise between the commercial AC power source and the PFC circuit, wherein the at least one fan motor inverter is mounted on a second substrate, the system controller is mounted on a third substrate, and the filter is mounted on a fourth substrate, wherein the fourth substrate is disposed below the third substrate, wherein the system controller controls the compressor inverter, the at least one fan motor inverter, and the PFC circuit.