To provide an electric power conversion system comprising a power module equipped with a switching element and a cooler for cooling the power module, which has a function of determining abnormal behaviour of the cooler.

The electric power conversion system comprises a cooler 3 comprising a heat sink 7 mounted on a power module 6, a blower 8 for blowing a refrigerant to the heat sink, a refrigerant temperature sensor 13 for measuring temperature of the refrigerant, a heat sink temperature sensor 14 for measuring temperature of the heat sink and a controller 5. The controller 5, when a switching element is turned off, calculates a first difference in temperature between the heat sink and the refrigerant at a first time and a second difference in temperature between the heat sink and the refrigerant at a second time following the first time. The controller 5, if the second temperature difference exceeds the first temperature difference, determines that abnormal behavior of the cooler.