A sample observation device, comprising: an illumination optical system and an observation optical system, wherein the illumination optical system and the observation optical system are disposed to be opposed, the illumination optical system includes a light source, a condenser lens and an aperture member, the observation optical system includes an objective lens and an imaging lens, the aperture member has a light-shielding part or a darkening part, and a transmission part, the aperture member is disposed so that the light-shielding part or the darkening part includes an optical axis of the illumination optical system, the transmission part is located outside of an outer edge of the light-shielding part or the darkening part, an image of the transmission part is formed at a pupil position of the objective lens, an image of an inner edge of the transmission part is formed inside of an outer edge of the pupil of the objective lens, and an image of an outer edge of the transmission part is formed outside of the outer edge of the pupil of the objective lens, and the following condition expression is satisfied: R ob − R 0 × β / R 1 × β − R ob < 1, where R 0 denotes a length from the optical axis of the observation optical system to the inner edge of the transmission part, R 1 denotes a length from the optical axis of the observation optical system to the outer edge of the transmission part, R ob denotes a radius of the pupil of the objective lens, and β denotes a value obtained by dividing a focal length of the objective lens by a focal length of the condenser lens. an illumination optical system and an observation optical system, wherein the illumination optical system and the observation optical system are disposed to be opposed, the illumination optical system includes a light source, a condenser lens and an aperture member, the observation optical system includes an objective lens and an imaging lens, the aperture member has a light-shielding part or a darkening part, and a transmission part, the aperture member is disposed so that the light-shielding part or the darkening part includes an optical axis of the illumination optical system, the transmission part is located outside of an outer edge of the light-shielding part or the darkening part, an image of the transmission part is formed at a pupil position of the objective lens, an image of an inner edge of the transmission part is formed inside of an outer edge of the pupil of the objective lens, and an image of an outer edge of the transmission part is formed outside of the outer edge of the pupil of the objective lens, and the following condition expression is satisfied: R ob − R 0 × β / R 1 × β − R ob < 1, where R 0 denotes a length from the optical axis of the observation optical system to the inner edge of the transmission part, R 1 denotes a length from the optical axis of the observation optical system to the outer edge of the transmission part, R ob denotes a radius of the pupil of the objective lens, and β denotes a value obtained by dividing a focal length of the objective lens by a focal length of the condenser lens. R 0 denotes a length from the optical axis of the observation optical system to the inner edge of the transmission part, R 1 denotes a length from the optical axis of the observation optical system to the outer edge of the transmission part, R ob denotes a radius of the pupil of the objective lens, and β denotes a value obtained by dividing a focal length of the objective lens by a focal length of the condenser lens.