High-temperature Young’s moduli of amorphous films in the SiO2-Y2O3 system were determined by using a newly developed measuring system for Young’s modulus. It was found that the modulus of amorphous SiO2 film at high temperature was close to that of silica glass. As Y2O3 was added to amorphous SiO2, the mean temperature coefficient of Young’s modulus of the amorphous films decreased monolonically from a positive value to a negative one. From this result, it was considered that yttrium ions act as a network modifier in the structure of amorphous films. In addition, from measurements of Young’s modulus at room temperature and its temperature coefficient, it is found that the difference in Young’s modulus due to the Y2O3 content observed at room temperature becomes extinguished with increasing temperature.