A voltage conversion device having a DC-DC converter which comprises a reactor which can temporarily accumulate energy using a direct current from a direct current power supply, and a switching element (Q1, Q2), and which converts, using the energy accumulated in the reactor, direct current power from the direct current power supply to a direct current power of a different voltage value utilizing a switching operation of the switching element (Q1, Q2) and outputs the converted power, a capacitor which is provided at an input side of the DC-DC converter and in parallel with the direct current power supply, and a cooling unit which cools the DC-DC converter with a cooling medium, the voltage conversion device comprising: a temperature sensor for detecting a temperature of the switching element (Q1, Q2); a switching frequency setting unit which sets a switching frequency of the switching element (Q1, Q2 to a higher first frequency (fh) or a second frequency (fl) which is lower than the first frequency (fh), based on the temperature of the switching element (Q1, Q2); and a switching controller which controls the switching operation of the switching element (Q1, Q2) by the set switching frequency, to control a voltage conversion ratio of the DC-DC converter; characterized by a cooling medium temperature detecting unit which detects a temperature of the cooling medium; wherein the cooling unit cools at least one of the reactor and the capacitor with the cooling medium; the switching frequency setting unit sets the switching frequency of the switching element (Q1, Q2) to the higher first frequency (fh) or the lower second frequency (fl), based on the temperature of the switching element (Q1, Q2), when the temperature of the cooling medium detected by the cooling medium temperature detecting unit is not higher than a set temperature T1; and the switching frequency setting unit sets the switching frequency of the switching element (Q1, Q2) to the higher first frequency (fh) regardless of the temperature of the switching element (Q1, Q2), when the temperature of the cooling medium detected by the cooling medium temperature detecting unit is higher than the set temperature T1. a temperature sensor for detecting a temperature of the switching element (Q1, Q2); a switching frequency setting unit which sets a switching frequency of the switching element (Q1, Q2 to a higher first frequency (fh) or a second frequency (fl) which is lower than the first frequency (fh), based on the temperature of the switching element (Q1, Q2); and a switching controller which controls the switching operation of the switching element (Q1, Q2) by the set switching frequency, to control a voltage conversion ratio of the DC-DC converter; characterized by a cooling medium temperature detecting unit which detects a temperature of the cooling medium; wherein the cooling unit cools at least one of the reactor and the capacitor with the cooling medium; the switching frequency setting unit sets the switching frequency of the switching element (Q1, Q2) to the higher first frequency (fh) or the lower second frequency (fl), based on the temperature of the switching element (Q1, Q2), when the temperature of the cooling medium detected by the cooling medium temperature detecting unit is not higher than a set temperature T1; and the switching frequency setting unit sets the switching frequency of the switching element (Q1, Q2) to the higher first frequency (fh) regardless of the temperature of the switching element (Q1, Q2), when the temperature of the cooling medium detected by the cooling medium temperature detecting unit is higher than the set temperature T1.