A power supply apparatus for an electric discharge machine configured to perform melt-removal on a workpiece by supplying pulse-like power to a machining gap between an electrode (1) and a workpiece (2). This power supply apparatus includes a second machining circuit configured to control a voltage to be applied to the machining gap and to cause a discharge in the machining gap, a first machining circuit adapted to operate to supply, when a discharge occurring in the machining gap in response to application of a voltage by the second machining circuit is detected in a first drive mode, a constant current to the machining gap, and also configured to operate to adapt a voltage source (14) in the second machining circuit to output a constant voltage in a second drive mode, and connection means (13) configured to close connection to a power supply (4) to supply electric power through the second machining circuit in the first mode and also configured to open the connection to the power supply (4) in the second mode so that the second machining circuit operates at a constant voltage supplied from the voltage source (14).