Recently, the demand for durability of optical thin films, which have long been used, has been growing as the performance of optical components improves. The stress of a film is an important parameter that is related to its adhesion. The electron beam (EB) and ion-assisted deposition (IAD) methods are widely used to fabricate optical thin films. However, there are few reports on long-term internal stress, despite the importance of this issue. Here we discuss the time dependence of the stress of ${\mathrm{SiO}}_2$ optical thin films in terms of optical characteristics in the infrared region. It was found that ${\mathrm{SiO}}_2$ thin films prepared by the EB and IAD methods exhibited compression stress. The Si–OH molecular bond was observed at around $930{\mathrm{cm}}^{-1}$ in the Fourier transform infrared spectroscopy spectrum of the sample prepared by the EB method, which exhibited a large change in internal stress after an elapsed time. It is considered that this change in bonding was related to the decrease in the stress of the films.