An SOI substrate evaluating method includes: forming a device onto a measuring SOI substrate, and previously determining a relationship between an interface state density and a leakage power upon application of radio-frequency thereon, or converting the interface state density to a resistance followed by previously determining a relationship between the converted resistance and the leakage power; measuring an interface state density of the evaluation target SOI substrate to determine the interface state density or a resistance converted from the interface state density; evaluating a leakage power of the evaluation target SOI substrate from the measured interface state density of the evaluation target SOI substrate on the basis of the determined relationship between the interface state density and the leakage power, or from a resistance converted from the measured interface state density of the evaluation target SOI substrate on the basis of the determined relationship between the resistance and leakage power.