in the detecting accuracy due to the decline of the output voltage of the phototransistor 8b resulting from the staining of the surface of the light emitting diode 8a or phototransistor 8b can be prevented in the case where the water is already supplied in the washer tank 1 the optical sensor is set with the current of the light emitting diode 8a employed in the previous operation in step 143 then in step 144 a constant current is fed to the light emitting diode 8a it is detected in step 145 whether the washing process is selected in the event that the washing process is not selected the flow proceeds to a succeeding process in step 146 for example rinsing process in the washing process if there is no water in the tank 1 the volume detecting device 20 detects the volume of laundries and the water is fed to a predetermined water level and thereafter the stirring vane 2 is rotated to produce the water stream the change in the output voltage ve of the phototransistor 8b after the start of stirring is indicated in the graph of fig4 in which lines a and b show the voltage ve change when a powder detergent is used and a line c indicates the change when a liquid detergent is used if washing is completed before a time point t1 e g the user sets the washing time period shorter than t1 the operation flow advances to a next process steps 147 and 148 in step 149 the output voltage ve is set to be ve1 at the.