A semiconductor processing system includes a gas supply system configured to supply water vapor into a process chamber that accommodates a target substrate. The gas supply system is provided with a gas generating apparatus for generating water vapor from purified water. The gas generating apparatus includes a first vaporizing section configured to spray the purified water along with a carrier gas and heat the purified water, so as to generate preparatory water vapor containing mist, and a second vaporizing section configured to vaporize mist contained in the preparatory water vapor, so as to generate process water vapor from the preparatory water vapor. In the second vaporizing section, a thin film having a mesh structure is disposed across a passage for the preparatory water vapor and configured to trap mist between the first vaporizing section and the process chamber.