To provide an eyecup device which is constituted of first and second cylindrical members that are freely slidably engaged each other, whose assembly can easily be performed and whose assembly is prevented from easily coming out after the assembly of the first and second cylindrical members is once completed.

A movable piece 26 which can elastically be deformed is integrally formed in the peripheral wall of the first cylindrical member 20 by removing a part of the material of the wall. A projecting part 28 is formed outside the movable piece. A guide groove 28' extending from the rear end edge of the second cylindrical member to the front end edge along the center axis of the inner wall face of the second cylindrical member 22 is formed on the inner wall face of the second cylindrical member 22. The guide groove freely slidably receives the projecting part. When the projecting part abuts on the trailing end of the guide groove, the movable piece receives force with which it is elastically deformed to the outside of the radius direction of the first cylindrical member.