The present invention provides an isolated switch-mode power supply device capable of sufficiently reducing power consumption in a standby mode. An isolated switch-mode power supply device includes: a capacitor that supplies control power for controlling switching of a switching element; a first control unit that includes a constant current supplying unit that controls switching of the switching element; a switching element that connects or disconnects the first control unit and the capacitor; a capacitance element unit having a capacitor to which a constant current is supplied from the constant current supplying unit, a capacitor charge voltage of the capacitance element unit changing according to an outputted voltage in the standby mode; and a second control unit that controls power supply to the first control unit by closing or opening the switching element during a switching pause period in the standby mode according to the capacitor charge voltage of the capacitor.