PURPOSE: To allow easy and low cost measurement of average temperature in an oil tank by providing a pressure gauge having a temperature scale for indicating the pressure variation due to temperature rise of oil while expressing in terms of temperature value.

CONSTITUTION: Volume of oil 6 in an oil tank 2 increases as the temperature of oil increases and thereby the scale of an oil level meter 5 fluctuates between low and high levels. Increase in the volume of oil 6 causes to compress the air 11 at the upper part in the oil tank 2 thus increasing the internal pressure. The increased pressure is then introduced through a pipe 9 to a pressure gauge 8 having temperature scale so that a pressure indicating pointer swings on the pressure and temperature scales. Average temperature of the oil 6 is measured based on the temperature determined by expressing the pressure rise due to the expansion of volumes of the oil 6 and the oil tank 6 in terms of temperature. This constitution allows direct read out of the temperature of oil in the oil tank from thermal expansion of the oil in the form of average temperature of oil.