An on-vehicle transformer comprising: a transformer main body; a tank that houses the transformer main body; an insulating oil filled into the tank to cool the transformer main body; a cooler that cools the insulating oil; a connection pipe that connects the tank and the cooler to each other; and a pump placed in the connection pipe to circulate the insulating oil between the tank and the cooler, wherein the cooler includes a heat exchange unit, and an air intake unit integrally connected to the heat exchange unit, the heat exchange unit has a plurality of oil flow portions and a plurality of air flow portions, the plurality of oil flow portions allowing the insulating oil to flow in one direction, each of the plurality of air flow portions being located between two of the plurality of oil flow portions adjacent to each of the plurality of air flow portions to allow outdoor air to flow toward an interior of the air intake unit in a direction that crosses an oil flow direction of the insulating oil in the plurality of oil flow portions, the air intake unit has an air intake fan located in the air intake unit to suction the outdoor air into the air intake unit through the air flow portions, characterized in that a sliding mechanism is provided between each of the plurality of oil flow portions and a corresponding one of the plurality of air flow portions adjacent to each of the plurality of oil flow portions, the sliding mechanism being configured to allow each of the plurality of air flow portions to slide in a direction along an air flow direction of the outdoor air in the plurality of air flow portions, and by sliding each of the plurality of air flow portions along the sliding mechanism, each of the plurality of air flow portions can move such that in the air flow direction, an edge of each of the plurality of air flow portions on an air intake unit side is located, toward an opposite side of the air intake unit side, at a distance from an edge of each of the plurality of oil flow portions on the opposite side of the air intake unit side, the sliding mechanism has a sliding contact portion and a rail portion, the sliding contact portion being provided on each of the plurality of oil flow portions, the rail portion being provided on each of the plurality of air flow portions so as to be in sliding contact with the sliding contact portion directly or indirectly, and extending in the direction along the air flow direction. a transformer main body; a tank that houses the transformer main body; an insulating oil filled into the tank to cool the transformer main body; a cooler that cools the insulating oil; a connection pipe that connects the tank and the cooler to each other; and a pump placed in the connection pipe to circulate the insulating oil between the tank and the cooler, wherein the cooler includes a heat exchange unit, and an air intake unit integrally connected to the heat exchange unit, the heat exchange unit has a plurality of oil flow portions and a plurality of air flow portions, the plurality of oil flow portions allowing the insulating oil to flow in one direction, each of the plurality of air flow portions being located between two of the plurality of oil flow portions adjacent to each of the plurality of air flow portions to allow outdoor air to flow toward an interior of the air intake unit in a direction that crosses an oil flow direction of the insulating oil in the plurality of oil flow portions, the air intake unit has an air intake fan located in the air intake unit to suction the outdoor air into the air intake unit through the air flow portions, characterized in that a sliding mechanism is provided between each of the plurality of oil flow portions and a corresponding one of the plurality of air flow portions adjacent to each of the plurality of oil flow portions, the sliding mechanism being configured to allow each of the plurality of air flow portions to slide in a direction along an air flow direction of the outdoor air in the plurality of air flow portions, and by sliding each of the plurality of air flow portions along the sliding mechanism, each of the plurality of air flow portions can move such that in the air flow direction, an edge of each of the plurality of air flow portions on an air intake unit side is located, toward an opposite side of the air intake unit side, at a distance from an edge of each of the plurality of oil flow portions on the opposite side of the air intake unit side, the sliding mechanism has a sliding contact portion and a rail portion, the sliding contact portion being provided on each of the plurality of oil flow portions, the rail portion being provided on each of the plurality of air flow portions so as to be in sliding contact with the sliding contact portion directly or indirectly, and extending in the direction along the air flow direction.