Image pickup optical system and image pickup unit of endoscope, and endoscope

An image pickup optical system of an endoscope comprising:

• a first optical member group that includes an objective optical member disposed closest to a subject side and an image-side optical member that is disposed adjacent to an image side of the objective optical member;
• a first lens barrel that holds the first optical member group;
• a second optical member group that is disposed on an image side of the first optical member group;
• and
• a second lens barrel that holds the second optical member group,
• wherein the objective optical member and the image-side optical member of the first optical member group are in contact with each other in a direction of an optical axis,
• the first and second lens barrels are fitted to each other in the direction of the optical axis so that the image-side optical member of the first optical member group is in contact with the second lens barrel, and
• a space is provided on an image side of an image-side end of the first lens barrel and on a subject side of a subject-side end of the second lens barrel in the direction of the optical axis in a state in which the image-side optical member of the first optical member group is in contact with the second lens barrel, and the first and second lens barrels are bonded to each other so as to be separable from each other.