PURPOSE: To suppress a noise by a method wherein the rotation of a motor is one-directional at the time of dehydration and, while the motor rotates to that direction, the ignition phase of a thyristor is so controlled as to reduce an effective voltage applied to the motor until the revolution of the motor reaches a predetermined value.

CONSTITUTION: At the time of a dehydration process, clutches 26 are coupled and an inner bath 19 is made to rotate. As the process is a centrifugal dehydration process, the rotation is one-directional. One of thyristors 11 and 12 is selected and the selected thyristor is ignited by an ignition circuit 14 to apply a current to a motor 4. In the one-directional rotation process, i.e., the dehydration process, the ignition circuit 14 actuates a current detection circuit 15 and, during a period from the start of the rotation to the time when the revolution of the motor reaches a steady certain value, the ignition phase of the selected thyristor, for instance the thyristor 11, is so controlled as to be delayed to suppress an effective voltage applied to the motor 4. When the revolution approaches the steady value, the delay of the ignition phase is reduced to apply a full voltage to the motor 4.