An optical element unit is insertable into and removable from an opening portion provided in an optical apparatus and a first portion with a first inner diameter and a second portion with a second inner diameter smaller than the first inner diameter are disposed inside of the opening portion. The optical element unit has a third portion with a first outer diameter and a fourth portion with a second outer diameter smaller than the first outer diameter, wherein the first outer diameter is greater than the second inner diameter. Therefore, when the optical element unit is inserted into the opening portion in a reverse way, the third portion contacts with the second portion so that the optical element cannot be fully inserted into the opening portion.