An in-vehicle transformation device comprising: - an iron core; - three or more windings (3, 4a, 4b) wound around the iron core and stacked on one another; - a plurality of insulating plates each disposed between adjacent windings of the three or more windings (3, 4a, 4b); - a tank configured to contain the iron core, the three or more windings (3, 4a, 4b), and the plurality of insulating plates and filled with insulation oil; - a plurality of spacers (62, 63, 72, 73) each located and interposed between a winding and an insulating plate facing each other among the three or more windings (3, 4a, 4b) and the plurality of insulating plates to provide a flow path for the insulation oil; - a pump configured to selectively cause the insulation oil to flow in one of one direction and another direction of directions orthogonal to a stacking direction of the three or more windings (3, 4a, 4b), the other direction being opposite to the one direction; and - a switching circuit configured to selectively incorporate, in an electric circuit, one of a part of the three or more windings and another part of the three or more windings, characterised by - a part of the plurality of spacers in contact with the part of the windings having a smaller flow resistance to the insulation oil flowing in the one direction than a flow resistance to the insulation oil flowing in the other direction, - another part of the plurality of spacers in contact with the other part of the windings having a smaller flow resistance to the insulation oil flowing in the other direction than a flow resistance to the insulation oil flowing in the one direction, - the pump being configured to cause the insulation oil to flow in the one direction when the part of the windings is incorporated in the electric circuit by the switching circuit, the pump being configured to cause the insulation oil to flow in the other direction when the other part of the windings is incorporated in the electric circuit by the switching circuit. - an iron core; - three or more windings (3, 4a, 4b) wound around the iron core and stacked on one another; - a plurality of insulating plates each disposed between adjacent windings of the three or more windings (3, 4a, 4b); - a tank configured to contain the iron core, the three or more windings (3, 4a, 4b), and the plurality of insulating plates and filled with insulation oil; - a plurality of spacers (62, 63, 72, 73) each located and interposed between a winding and an insulating plate facing each other among the three or more windings (3, 4a, 4b) and the plurality of insulating plates to provide a flow path for the insulation oil; - a pump configured to selectively cause the insulation oil to flow in one of one direction and another direction of directions orthogonal to a stacking direction of the three or more windings (3, 4a, 4b), the other direction being opposite to the one direction; and - a switching circuit configured to selectively incorporate, in an electric circuit, one of a part of the three or more windings and another part of the three or more windings, characterised by - a part of the plurality of spacers in contact with the part of the windings having a smaller flow resistance to the insulation oil flowing in the one direction than a flow resistance to the insulation oil flowing in the other direction, - another part of the plurality of spacers in contact with the other part of the windings having a smaller flow resistance to the insulation oil flowing in the other direction than a flow resistance to the insulation oil flowing in the one direction, - the pump being configured to cause the insulation oil to flow in the one direction when the part of the windings is incorporated in the electric circuit by the switching circuit, the pump being configured to cause the insulation oil to flow in the other direction when the other part of the windings is incorporated in the electric circuit by the switching circuit.