A thermal sensor in which, when an object to be measured is a water-based liquid, attachment of air bubbles to the external surface of the sensor is reduced to improve measurement accuracy. The thermal sensor has a sensing element (21a) including a heat producing body and a temperature sensing body, a resin mold (23) for sealing the sensing element (21a), and a heat transmission member (21c) for transmitting heat between the sensing element (21a) and a water-based object to be measured. A part of the heat transmission member (21c) is exposed from the resin mold (23) to form an exposed surface section. A hydrophilic film (50) formed of a silicon oxide film is applied to the exposed surface section and to that part of the surface of the resin mold which is positioned around the exposed surface section.