In this proposal report, we clarify the band alignment at the interface between Ga₂O₃ and SiO₂, which serves as a gate dielectric, paying particular attention to the effect of heat treatment. The band alignment is a critical parameter that determines the threshold voltage, which is a guide to the gate voltage required for switching metal/oxide/semiconductor field-effect transistors (MOSFETs), and the gate leakage current level, which determines the reliability of the gate dielectric film. The changes in band alignment with heat treatment were investigated by two methods: analyzing ultraviolet photoelectron spectroscopy of a high-quality β-Ga₂O₃ singlecrystal epitaxial wafer surface, and analyzing the X-ray photoelectron spectroscopy of the annealed stacks with SiO₂ deposition on the wafer.