An electric power converter, comprising: a non-isolated buck-boost converter circuit configured to output, by increasing or decreasing a voltage being inputted thereinto, a DC voltage; an isolated converter circuit configured to output a DC voltage to a load by inputting a DC voltage outputted from the non-isolated buck-boost converter circuit; and a control unit configured to control the non-isolated buck-boost converter circuit and the isolated converter circuit, wherein the isolated converter circuit is a resonance type converter which includes a transformer having a primary winding and a secondary winding; a third semiconductor switching device, a fourth semiconductor switching device, a resonance capacitor and a resonance reactor connected to the primary winding; and a rectifier circuit, and a smoothing capacitor connected to the secondary winding; and wherein the isolated converter circuit is an LLC converter, wherein the control unit is configured to operate the LLC converter at a fixed switching frequency, and wherein the control unit is configured to control a time of half a period of a fixed switching frequency in the LLC converter to be equal to a time in which a dead time is added to half a period of a series resonance frequency determined between the resonance capacitor and the resonance reactor, wherein the electric power converter comprises a temperature sensor, wherein the control unit is configured to store, as a function, a value of the series resonance frequency which changes due to a temperature change, and the control unit is configured to receive at its input a temperature detected by the temperature sensor, wherein the control unit is configured to alter the fixed switching frequency in accordance with a monitored value of the temperature sensor. a non-isolated buck-boost converter circuit configured to output, by increasing or decreasing a voltage being inputted thereinto, a DC voltage; an isolated converter circuit configured to output a DC voltage to a load by inputting a DC voltage outputted from the non-isolated buck-boost converter circuit; and a control unit configured to control the non-isolated buck-boost converter circuit and the isolated converter circuit, wherein the isolated converter circuit is a resonance type converter which includes a transformer having a primary winding and a secondary winding; a third semiconductor switching device, a fourth semiconductor switching device, a resonance capacitor and a resonance reactor connected to the primary winding; and a rectifier circuit, and a smoothing capacitor connected to the secondary winding; and wherein the isolated converter circuit is an LLC converter, wherein the control unit is configured to operate the LLC converter at a fixed switching frequency, and wherein the control unit is configured to control a time of half a period of a fixed switching frequency in the LLC converter to be equal to a time in which a dead time is added to half a period of a series resonance frequency determined between the resonance capacitor and the resonance reactor, wherein the electric power converter comprises a temperature sensor, wherein the control unit is configured to store, as a function, a value of the series resonance frequency which changes due to a temperature change, and the control unit is configured to receive at its input a temperature detected by the temperature sensor, wherein the control unit is configured to alter the fixed switching frequency in accordance with a monitored value of the temperature sensor.