An inverted microscope comprising: an objective lens; an imaging lens configured to form an image of light which passes through the objective lens from a specimen; and a microscope main body (10, 10a, 10b) in which a plurality of optical devices (20, 20A, 20B, 20C, 20D, 20E) are configured to be arranged between the objective lens and the imaging lens in a direction of an optical axis (La) of the objective lens and which includes therein a plurality of installation units (13, 13') that allow the plurality of optical devices (20, 20A, 20B, 20C, 20D, 20E) to be fixed independently of one another, each of the plurality of installation units (13, 13') including an abutting reference surface (133, 401a) on which any one of the plurality of optical devices (20, 20A, 20B, 20C, 20D, 20E) is configured to be abutted, wherein each of the optical devices (20, 20A, 20B, 20C, 20D, 20E) is installed in a respective installation unit (13, 13') and includes at least one optical splitting mirror (21, 21a, 21b, 21c, 21d); the installation units (13, 13') are configured to have the same shape, characterised in that the installation units (13, 13') are arranged on respective stages at positions shifted from each other in a direction perpendicular to the optical axis (La) of the objective lens so that, when the plurality of optical devices (20, 20A, 20B, 20C, 20D, 20E) is installed in the installation units (13, 13'), optical paths of an incident-light-illumination optical system of the respective stages are shifted from each other; the abutting reference surfaces (133, 401a) of the plurality of the installation units (13, 13') are shifted from one another in a direction perpendicular to the optical axis (La) of the objective lens, and the imaging lens is arranged at a position where the optical axis (Lb) of the imaging lens is shifted by a shift amount (δ3) from the optical axis (La) of the objective lens, the shift amount (δ3) being half of the total shift amount (Δ) of an observation light beam in the direction perpendicular to the optical axis (La) of the objective lens when the observation light beam from the objective lens passes through the plurality of optical devices (20, 20A, 20B, 20C, 20D, 20E). an objective lens; an imaging lens configured to form an image of light which passes through the objective lens from a specimen; and a microscope main body (10, 10a, 10b) in which a plurality of optical devices (20, 20A, 20B, 20C, 20D, 20E) are configured to be arranged between the objective lens and the imaging lens in a direction of an optical axis (La) of the objective lens and which includes therein a plurality of installation units (13, 13') that allow the plurality of optical devices (20, 20A, 20B, 20C, 20D, 20E) to be fixed independently of one another, each of the plurality of installation units (13, 13') including an abutting reference surface (133, 401a) on which any one of the plurality of optical devices (20, 20A, 20B, 20C, 20D, 20E) is configured to be abutted, wherein each of the optical devices (20, 20A, 20B, 20C, 20D, 20E) is installed in a respective installation unit (13, 13') and includes at least one optical splitting mirror (21, 21a, 21b, 21c, 21d); the installation units (13, 13') are configured to have the same shape, characterised in that the installation units (13, 13') are arranged on respective stages at positions shifted from each other in a direction perpendicular to the optical axis (La) of the objective lens so that, when the plurality of optical devices (20, 20A, 20B, 20C, 20D, 20E) is installed in the installation units (13, 13'), optical paths of an incident-light-illumination optical system of the respective stages are shifted from each other; the abutting reference surfaces (133, 401a) of the plurality of the installation units (13, 13') are shifted from one another in a direction perpendicular to the optical axis (La) of the objective lens, and the imaging lens is arranged at a position where the optical axis (Lb) of the imaging lens is shifted by a shift amount (δ3) from the optical axis (La) of the objective lens, the shift amount (δ3) being half of the total shift amount (Δ) of an observation light beam in the direction perpendicular to the optical axis (La) of the objective lens when the observation light beam from the objective lens passes through the plurality of optical devices (20, 20A, 20B, 20C, 20D, 20E).