A dehydrating apparatus of an automatic washing machine includes: an unbalance sensor mounted on a controller mounted into a top cover, for sensing an unbalance of a washing tub during performing a dehydrating function; a magnet mounted on a door, for sensing an opening of the door during performing the dehydrating function; and a hall sensor mounted on an another controller more adjacent to the magnet, for sensing switch on or off state by means of a magnetic field of the magnet in the event of opening or closing of the door. To sense the unbalance of the washing tub for a certain time after performing the dehydrating cycle, sensing time of the unbalance is counted from starting time of the dehydrating function to compare with a certain time. If the sensing time of the unbalance is more than a certain time, a sub-routine for sensing the unbalance. If not, the sensing time of the unbalance is accumulatively counted and stored into a memory, to determine the dehydrating cycle time. Thus, the unbalance of the washing tub and the opening of the door can exactly be sensed to control them.