PURPOSE: To detect oil in water with simple constitution and high sensitivity by holding the temp. of a heat sensitive semiconductor element immersed in the oil to be measured in a measuring tank at a set temp.

CONSTITUTION: The temp. of a heat sensitive semiconductor element RT is maintained roughly at 110W120°C by controlling the feedback rate of an operational amplifier OA to a suitable rate. At every sticking and evaporation of the water in the oil to be measured on the surface of the element RT, the temp. of the element RT drops and the resistance value thereto increases. As a result, the potential of the terminal (b) of an electric bridge circuit BR increases, the output and feedback rate of the amplifier OA increase the temp. of the element RT rises, thus restoring the set temp. The output of the amplifier OA passes through a high pass filter HPF which eliminates the fluctuating components change of the agitating speed by convection of the oil to be measured and the temp. of the heat sensitive semiconductor element. After rectification, this outut is integrated and the result is indicated on an indicator.