An objective optical system comprising: a convex reflection portion which reflects light traveling toward a sample; a concave mirror which comprises a concave reflection portion which reflects light reflected by the convex reflection portion toward the sample; a cover member (103, 120) which is disposed on an optical path of light reflected by the concave reflection portion, the cover member (103, 120) having an incidence surface (103a, 120a) and an emission surface (103b, 120b), the incidence surface (103a, 120a) being a surface on which light reflected by the concave reflection portion is incident, the emission surface (103b, 120b) being a surface through which light incident on the incidence surface (103a, 120a) is emitted, the emission surface (103b, 120b) serving as a liquid contact surface coming into contact with liquid (WT, CF) interposed between the liquid contact surface and the sample, the emission surface (103b, 120b) being formed to be orthogonal to the optical path of light reflected by the concave reflection portion; a lens barrel which internally supports the concave mirror; a liquid holding member which supports the cover member such that an internal space is partitioned by the cover member (103, 120), the liquid holding member having a first end portion and a second end portion, the first end portion being attached to an end portion of the lens barrel on an object side, the liquid holding member being capable of holding liquid (WT, CF) in an internal space on the second end portion side partitioned by the cover member (103, 120) such that liquid (WT, CF) is interposed between the cover member (103, 120) and the sample disposed at the second end portion side; and a liquid supply portion which supplies liquid (WT, CF) to an internal space on the second end portion side partitioned by the cover member (103, 120) from a side surface of the liquid holding member, wherein the convex reflection portion is formed in a middle portion on the incidence surface (103a, 120a) of the cover member (103, 120), or a convex mirror having the convex reflection portion is attached to the middle portion. a convex reflection portion which reflects light traveling toward a sample; a concave mirror which comprises a concave reflection portion which reflects light reflected by the convex reflection portion toward the sample; a cover member (103, 120) which is disposed on an optical path of light reflected by the concave reflection portion, the cover member (103, 120) having an incidence surface (103a, 120a) and an emission surface (103b, 120b), the incidence surface (103a, 120a) being a surface on which light reflected by the concave reflection portion is incident, the emission surface (103b, 120b) being a surface through which light incident on the incidence surface (103a, 120a) is emitted, the emission surface (103b, 120b) serving as a liquid contact surface coming into contact with liquid (WT, CF) interposed between the liquid contact surface and the sample, the emission surface (103b, 120b) being formed to be orthogonal to the optical path of light reflected by the concave reflection portion; a lens barrel which internally supports the concave mirror; a liquid holding member which supports the cover member such that an internal space is partitioned by the cover member (103, 120), the liquid holding member having a first end portion and a second end portion, the first end portion being attached to an end portion of the lens barrel on an object side, the liquid holding member being capable of holding liquid (WT, CF) in an internal space on the second end portion side partitioned by the cover member (103, 120) such that liquid (WT, CF) is interposed between the cover member (103, 120) and the sample disposed at the second end portion side; and a liquid supply portion which supplies liquid (WT, CF) to an internal space on the second end portion side partitioned by the cover member (103, 120) from a side surface of the liquid holding member, wherein the convex reflection portion is formed in a middle portion on the incidence surface (103a, 120a) of the cover member (103, 120), or a convex mirror having the convex reflection portion is attached to the middle portion.