In a DC-DC voltage conversion device, overheat of a diode element connected in anti-parallel with a semiconductor switching element is a problem, and in order to resolve this, a temperature of a semiconductor switching element of a main conversion circuit is detected, a temperature of a diode element connected in parallel with the semiconductor switching element is calculated using a correction calculation of the detected semiconductor switching element temperature value in accordance with a step-up ratio of the DC-DC voltage conversion device, and diode element overheat protection is carried out in accordance with the calculated temperature value.