To reduce a stop time of transportation of wafers which occurs when one stepper handles many kinds of products, before exposure of semiconductor wafers of a first cassette port 7a is completed, a process recipe for semiconductor wafers of a second cassette port 7b is obtained from a host computer 2, the progress of the exposure of the semiconductor wafers of the first cassette port 7a is detected via a sequencer 5, it is determined, based on the obtained process recipe, whether or not the semiconductor wafers of the second cassette port 7b can be transported to an exposure stage following the last semiconductor wafer of the first cassette port 7a, and a stepper 1 is caused to perform exposure in accordance with the determination result and the progress detection result.