PURPOSE: To judge the degree of damage of the laundry by detecting the light transmittance of washing water, detecting the extreme reduction of the obtained transmittance signal, and judging the waste thread quantity in the washing water.

CONSTITUTION: At the time t1 shortly after the start of 'washing', the number n1 of the times when the light transmittance signal sent from a transmittance sensor is the preset voltage V' or below is counted in the period T. Only mud and sand are removed from the laundry into washing water, waste threads are rarely removed, and the number n1 of the times when the transmittance signal is the preset voltage V' or below is small. At the time t2 some time later, the number n2 of the times when the transmittance signal is the preset voltage V' or below is likewise counted in the period T. The quantity of mud and sand is not increased at this time, and the waste thread quantity is increased by the damage of the laundry. The number n2 of the times when the transmittance signal is the preset voltage V' or below is increased. When n=n2-n1 is calculated, the waste thread quantity contained in the washing water can be calculated, and it can be regarded as the degree of damage of the laundry.