A substrate assembly including a photosensitive etching glass substrate; and a first substrate and a second substrate for interposing both main surfaces of the photosensitive etching glass substrate between them. One of the main surfaces of the photosensitive etching glass substrate is thermally bonded to the first substrate, and the other main surface of the photosensitive etching glass substrate is bonded to the second substrate. When a thermal expansion coefficient of the photosensitive etching glass substrate is defined as C0, and a thermal expansion coefficient of the first substrate is defined as C1, and a thermal expansion coefficient of the second substrate is defined as C2, C1/C2 satisfies a relation of 0.7 or more and 1.3 or less, and at least one of a relation of C0/C1 satisfying less than 0.7 or larger than 1.3, and a relation of C0/C2 satisfying less than 0.7 or larger than 1.3 is satisfied.