A wet processing apparatus includes a bath in which a plurality of semiconductor substrates are to be seated, and which bath has drain valves defining drain openings between a bottom wall and the lower portions of side walls of the bath. Each of the drain valves also includes a gate mounted on the lower portion of one of the side walls. The gates of the drain valves are movable to selectively open and close the drain openings. A plurality of substrates are transferred into the bath and are held under a body of liquid contained in a bath while the substrates are oriented vertically as disposed generally parallel to one another. In this state, the side walls of the baths face the peripheral edges of the substrates at both sides of the substrates. Thus, the substrates are prevented from moving toward and becoming stuck to one another when the liquid is drained from the bath.