A mirror unit comprising: a mirror device that includes a base including a first surface and a second surface opposite to the first surface, and a movable mirror supported in the base to be movable along a first direction that intersects the first surface; an optical function member including a third surface that faces the second surface, and a fourth surface opposite to the third surface; a fixed mirror that is disposed on a side opposite to the mirror device with respect to the optical function member; and i) a light passage portion provided in the mirror device that is configured to constitute a first part of an optical path between a beam splitter unit disposed on one side of the mirror unit in said first direction that constitutes an interference optical system in combination with the movable mirror and the fixed mirror, wherein the optical function member is provided with a light transmitting portion that faces the light passage portion in the first direction and that is configured to constitute a second part of the optical path between the beam splitter unit and the fixed mirror, or ii) a light passage portion being not provided in the mirror device and the optical function member is provided with a light transmitting portion in a portion that does not overlap the mirror device in the first direction and is configured to constitute a part of the optical path between the beam splitter unit and the fixed mirror; the light transmitting portion is a portion that is configured to correct an optical path difference that occurs between an optical path between the beam splitter unit and the movable mirror, and the optical path between the beam splitter unit and the fixed mirror, and the second surface of the base and the third surface of the optical function member are joined to each other. a mirror device that includes a base including a first surface and a second surface opposite to the first surface, and a movable mirror supported in the base to be movable along a first direction that intersects the first surface; an optical function member including a third surface that faces the second surface, and a fourth surface opposite to the third surface; a fixed mirror that is disposed on a side opposite to the mirror device with respect to the optical function member; and i) a light passage portion provided in the mirror device that is configured to constitute a first part of an optical path between a beam splitter unit disposed on one side of the mirror unit in said first direction that constitutes an interference optical system in combination with the movable mirror and the fixed mirror, i) a light passage portion provided in the mirror device that is configured to constitute a first part of an optical path between a beam splitter unit disposed on one side of the mirror unit in said first direction that constitutes an interference optical system in combination with the movable mirror and the fixed mirror, wherein the optical function member is provided with a light transmitting portion that faces the light passage portion in the first direction and that is configured to constitute a second part of the optical path between the beam splitter unit and the fixed mirror, or ii) a light passage portion being not provided in the mirror device and the optical function member is provided with a light transmitting portion in a portion that does not overlap the mirror device in the first direction and is configured to constitute a part of the optical path between the beam splitter unit and the fixed mirror; the light transmitting portion is a portion that is configured to correct an optical path difference that occurs between an optical path between the beam splitter unit and the movable mirror, and the optical path between the beam splitter unit and the fixed mirror, and the second surface of the base and the third surface of the optical function member are joined to each other.