To monitor moisture in a chamber at a film deposition time without exerting an influence at all on a chamber atmosphere.

In a detection method of a moisture content in the chamber in a film deposition device having the chamber for performing film deposition on a substrate, detection of the moisture content includes a process for introducing an electromagnetic wave having a frequency in a terahertz domain into the chamber, and a process for detecting the introduced electromagnetic wave, and the moisture content is detected based on a spectrum peak of water of the detected electromagnetic wave.