A method of manufacturing a semiconductor device includes supplying a process gas into a process vessel accommodating a substrate to form a thin film on the substrate and supplying a cleaning gas into the process vessel to clean an inside of the process vessel, after the supplying the process gas to form the thin film is performed a predetermined number of times. The projector may include at least one cylindrical lens or at least one pair of X-Y scanners. The frequency determiner determines a frequency for determining a time period of a detecting operation to detect differential quantum efficiency of the laser diode.