An electric power conversion device supplies electric power of a DC power source from an input part to an output part through a power conversion circuit having a transformer and switches. The transformer has primary and secondary coils magnetically connected. The device has a choke coil, an auxiliary coil and a control part. The auxiliary coil is connected to the output part and magnetically connected to the choke coil. The auxiliary coil is wound to allow a current to flow from a negative-electrode side to a positive-electrode side of the output part when a current flows from the power source to the choke coil. The control part performs a switching control of the switches to supply electric power of the source to the output part through the auxiliary coil to prevent generation of magnetic flux in the primary coil and the current from flowing in the secondary coil.