An optical unit having a distal end, a proximal end, and an optical axis in a first axial direction, the optical unit comprising: a cover glass having an incidence surface on the distal end side, an emission surface on the proximal end side, and a side surface; an optical lens disposed on the emission surface side of the cover glass; a holding tube having a cover glass fixing part that fixes the side surface of the cover glass, a cover glass position restricting part that restricts a position of the cover glass in the first axial direction, and an optical lens position restricting part that restricts a position of the optical lens in the first axial direction; and a spacer (144,178) that is disposed between the cover glass and the cover glass position restricting part, has a first end surface (144A, 178A) and a second end surface (144B,178B) at both ends thereof in the first axial direction, and has a window part serving as an optical channel in the first axial direction between the first end surface and the second end surface, the first end surface being a surface disposed on the cover glass side, an external diameter (D1,D5,D7) of the first end surface being smaller than an internal diameter (D2,D6,D8) of the cover glass fixing part, and an antireflection treatment layer being provided on at least the first end surface; wherein the holding tube includes a first recess that has a first inner wall surface and a first bottom surface, and accommodates the cover glass, and a second recess, that has a second inner wall surface and a second bottom surface, is open to the first bottom surface of the first recess, and accommodates the spacer, wherein the first inner wall surface is provided at a position that faces the side surface of the cover glass and has the cover glass fixing part, wherein the second bottom surface is provided at position that faces the second end surfaces of the spacer and has the cover glass position restricting part, wherein an internal diameter of the second inner wall surface is made smaller than an internal diameter of the first inner wall surface, and a depth of the second recess in the first axial direction is made smaller than a thickness of the spacer in the first axial direction, and wherein a solder pool part is provided in a region surrounded by the first recess, the cover glass, and the spacer. a cover glass having an incidence surface on the distal end side, an emission surface on the proximal end side, and a side surface; an optical lens disposed on the emission surface side of the cover glass; a holding tube having a cover glass fixing part that fixes the side surface of the cover glass, a cover glass position restricting part that restricts a position of the cover glass in the first axial direction, and an optical lens position restricting part that restricts a position of the optical lens in the first axial direction; and a spacer (144,178) that is disposed between the cover glass and the cover glass position restricting part, has a first end surface (144A, 178A) and a second end surface (144B,178B) at both ends thereof in the first axial direction, and has a window part serving as an optical channel in the first axial direction between the first end surface and the second end surface, the first end surface being a surface disposed on the cover glass side, an external diameter (D1,D5,D7) of the first end surface being smaller than an internal diameter (D2,D6,D8) of the cover glass fixing part, and an antireflection treatment layer being provided on at least the first end surface; wherein the holding tube includes a first recess that has a first inner wall surface and a first bottom surface, and accommodates the cover glass, and a second recess, that has a second inner wall surface and a second bottom surface, is open to the first bottom surface of the first recess, and accommodates the spacer, wherein the first inner wall surface is provided at a position that faces the side surface of the cover glass and has the cover glass fixing part, wherein the second bottom surface is provided at position that faces the second end surfaces of the spacer and has the cover glass position restricting part, wherein an internal diameter of the second inner wall surface is made smaller than an internal diameter of the first inner wall surface, and a depth of the second recess in the first axial direction is made smaller than a thickness of the spacer in the first axial direction, and wherein a solder pool part is provided in a region surrounded by the first recess, the cover glass, and the spacer.