In a power conversion device including: a converter which steps up or down a voltage of a direct current power supply; and an inverter which converts the direct current voltage obtained by the converter into an alternating voltage to drive an electric motor, a plurality of magnetic parts are arranged above a switching element assembly unit with a water-cooled type second heat sink interposed between the switching element assembly unit and the plurality of magnetic parts, the switching element assembly unit configured by mounting all the switching element modules on upper and lower surfaces of a water-cooled type first heat sink. Accordingly, it is possible to provide a power conversion device capable of housing many switching element modules in a compact space and cooling them effectively, while preventing the influence of a noise due to the magnetic parts from acting on the switching element modules as much as possible.