High-performance hydrogen gas sensors are needed to ensure safe use of hydrogen gas as a clean energy resource. Pt catalyst-loaded tungsten oxide (Pt/WO3), which shows gasochromism, is an excellent candidate for optical hydrogen gas sensor applications. Pt/WO3 turns blue in a hydrogen gas atmosphere and its electrical conductivity also changes at the same time. This study investigated the influence of partial pressures of hydrogen and oxygen gases on the gasochromism of Pt/WO3. Using elementary gasochromic reactions as a basis, we estimated the concentration of hydrogen injected into a WO3 lattice from the equilibrium of elementary reaction velocities. The estimation suggested that the inverse of the hydrogen concentration in WO3 was inversely proportional to the partial pressure of hydrogen and was proportional to that of oxygen. Additionally, the estimation agreed well with the results of Pt/WO3 gasochromic properties. Therefore, we were able to confirm the dependence of gasochromism on the partial pressures of hydrogen and oxygen gases, and substantiate that Pt/WO3 is capable of detecting hydrogen gas concentrations in various atmosphere conditions where oxygen gas partial pressures change.