Provided is a power supply apparatus for sub-modules of a Modular Multilevel Converter (MMC) which stably supplies power to the sub-modules of the MMC in connection with an HVDC system. The power supply apparatus for sub-modules of an MMC can include a charging unit in which an input voltage between P and N busses of the MMC is stored, a relay unit connected in parallel with the charging unit, a resistor connected in series with the relay unit, a TVS diode connected in series with the resistor, a Zener diode connected in series with the TVS diode, a transformer for delivering the input voltage (in a primary winding) to a secondary winding thereof, and a switch for switching the flow of current supplied to the transformer.