PURPOSE: To know the concentration of a detergent by detecting the electric resistance value of the detergent which is dissolved in water.

CONSTITUTION: The needle 4 of a meter 3 is connected to a bimetal 2 wound with a heat wire 1. One terminal of the heat wire 1 is connected to an electrode 9 installed in the tub of a washing machine 8, and the other terminal is connected to a power source 6 together with an electrode 11 installed in the tub of the washing machine 8. When the amount of a detergent thrown in the washing machine 8 is much, the electric resistance between the electrodes 9 and 11 is small and heat wire 1 generates heat; the bimetal 2 curves and the needle 4 is deflected to indicate the concentration of the detergent. Consequently, even when the meter 3 is installed on the console panel of the washing machine 8 which oscillates greately, the concentration of the detergent is known securely and the advance state of washing is also known.