An on-vehicle transformer comprising: a transformer body ; a tank to house the transformer body ; an insulating oil filled into the tank to cool the transformer body ; a cooler to cool the insulating oil ; a connection pipe to connect the tank to the cooler ; a pump placed in the connection pipe to circulate the insulating oil between the tank and the cooler ; and characterized in that an oil flow relay being placed on a discharge portion side of the pump in the connection pipe , the oil flow relay including: a blade member arranged in the connection pipe and supported so as to be rotatable about a central axis (C) orthogonal to the connection pipe ; a movable contactor having a movable contact and rotating about the central axis (C) with rotation of the blade member ; and a fixed contact located on a rotation path of the movable contact , wherein the oil flow relay is configured such that an operating flow rate in the oil flow relay becomes smaller as a temperature of the insulating oil becomes lower, the operating flow rate being a flow rate of the insulating oil flowing through the connection pipe when the movable contact and the fixed contact come into contact with each other, wherein the blade member has a shaft portion extending on the central axis (C) and is supported by a bearing portion that is in sliding contact with the shaft portion , and the shaft portion is made of a material having a thermal expansion coefficient higher than that of the bearing portion . a transformer body ; a tank to house the transformer body ; an insulating oil filled into the tank to cool the transformer body ; a cooler to cool the insulating oil ; a connection pipe to connect the tank to the cooler ; a pump placed in the connection pipe to circulate the insulating oil between the tank and the cooler ; and characterized in that an oil flow relay being placed on a discharge portion side of the pump in the connection pipe , the oil flow relay including: a blade member arranged in the connection pipe and supported so as to be rotatable about a central axis (C) orthogonal to the connection pipe ; a movable contactor having a movable contact and rotating about the central axis (C) with rotation of the blade member ; and a fixed contact located on a rotation path of the movable contact , wherein a blade member arranged in the connection pipe and supported so as to be rotatable about a central axis (C) orthogonal to the connection pipe ; a movable contactor having a movable contact and rotating about the central axis (C) with rotation of the blade member ; and a fixed contact located on a rotation path of the movable contact , wherein the oil flow relay is configured such that an operating flow rate in the oil flow relay becomes smaller as a temperature of the insulating oil becomes lower, the operating flow rate being a flow rate of the insulating oil flowing through the connection pipe when the movable contact and the fixed contact come into contact with each other, wherein the blade member has a shaft portion extending on the central axis (C) and is supported by a bearing portion that is in sliding contact with the shaft portion , and the shaft portion is made of a material having a thermal expansion coefficient higher than that of the bearing portion . <EOS>