A method of manufacturing a semiconductor device includes first and second processes, the latter requiring more processing time. An apparatus for performing the semiconductor manufacturing process includes a first reactor, and a plurality of second reactors for each first reactor. A first group of wafers are subjected to the first process within the first reactor, and are then transferred into a second reactor as isolated from the outside air. The first group of wafers is subjected to the second process within the second reactor. At the same time, a second group of wafers are subjected to the first process within the first reactor. After the first process is completed, the second group of wafers is transferred into an unoccupied one of the second reactors as isolated from the outside air. There, the second group of wafers is subjected to the second process. Accordingly, process failures otherwise due to the exposure of the wafers are minimized, and productivity is high despite the difference in the processing times.