Provided is a capacitor discharge device that quickly and safely discharges a capacitor charged with energy inside a sub-module of an MMC converter, the device including: a capacitor storing a DC voltage inside a sub-module of an MMC converter; a power supply unit supplying operating power required in the submodule by using the voltage stored in the capacitor; a first resistor connected in parallel to the capacitor; a second resistor having a lower resistance value and a larger heat capacity than the first resistor so as to rapidly discharge the capacitor storing the voltage; a first switching contact connecting and disconnecting the capacitor and the second resistor; a switching unit operating switching of the first switching contact by the operating power; a second switching contact connecting and disconnecting the power supply unit and the switching unit; and a control unit operating switching of the second switching contact.