• Development of humidity-controlled spin-coating system using N 2 gas. • Variation of the film thickness in presence or absence of N 2 gas. • Doubled film thickness obtained in high humidity compared to low humidity. Spin coating has been used for various applications, such as the coating of resist solutions in semiconductor processes and the fabrication of functional thin films. By rotating at a high speed, thin films can be formed by centrifugal force. Since the film thickness can be controlled by the number of rotations and time, spin coating is often used for simple experiments at the laboratory. However, in a normal laboratory environment, i.e. not a clean room, temperature and humidity can vary depending on the season, time of day, and weather. In this study, we attempted to fabricate electrochromic WO 3 thin films using a spin coater, which we set up independently, that can control humidity. Compared to the WO 3 thin film deposited directly during the rainy season in Japan, the film thickness was doubled by controlling the humidity, even when deposited under the same spin-coating conditions. The WO 3 thin films prepared by this humidity-controlled process exhibited favorable electrochromic properties. Because, especially in systems containing volatile solvents, it was possible to control the film thickness and function during deposition by controlling the humidity, and the results of this research provide a reference to develop a convenient method for obtaining functional thin films regardless of the weather or season.