To detect fluctuation in the quality or crystallinity of a thin film due to moisture by providing a reactor, or a part coupled therewith, a moisture gauge for measuring the moisture in a reaction gas.

A moisture gauge 4 is fixed to the inlet part of a gas discharging means 3 communicating with a reactor 1 in order to measure the moisture in a reaction gas at the time of processing a wafer 6. The moisture gauge 4 is fixed to the joint of a gas introduction part 2 to the reactor 1 or a part communicating with the reactor 1 directly. Moisture in the reaction gas is measured by the moisture gauge 4 during or after processing of the wafer 6. According to the arrangement, generation of a defective wafer can be prevented by detecting fluctuation in the quality or crystallinity of a thin film due to moisture in the reaction gas during process.