An optical device is disposed with: a swollen portion that is attached to the underside of a reflective surface of a reflective mirror, swells in an outward direction of the reflective mirror and is configured as part of a spherical surface. A mirror orientation adjustment lever that is attached to the underside of the reflective surface of the reflective mirror, extends along the underside of the reflective mirror, and protrudes from an end edge of the reflective mirror when seen planarly. The reflective mirror is disposed along a side wall of an optical parts-use casing. The side wall includes, at the position where the reflective mirror is disposed, a spherical concave portion corresponding to a spherical surface of the swollen portion, and when the reflective mirror is disposed at the disposition position, the swollen portion is supported by the spherical concave portion.