To enable a tumbler receiving seat to be positioned and then simply fixed on the inner surface of a case element without the need for welding.

In an assembly structure of a case element of a lock case and a tumbler receiving seat, the plated case element of the lock case includes an insertion opening into which the rear end of a cylinder tube or a thumb-turn tube is inserted, small projections and locking portions extending in the direction perpendicular to the direction in which the small projections project. On the other hand, the tumbler receiving seat is likewise plated and fixed on the inner wall surface of the case element so as to be positioned in the insertion opening, and the tumbler receiving seat includes a receiving portion having a tumbler fitting hole, an engagement projection piece projecting from an opposite wall portion on the one side of the receiving portion and a positioning projection piece projecting from an opposite wall portion on the other side of the same receiving portion. When fixing the tumbler receiving seat to the case element, the engagement projection pieces are engageably inserted in the locking portions and then the positioning projection piece is fixed by caulking with the positioning projection piece jointed onto the inner wall surface of the case element via positioning means provided in the positioning projection piece and the case element.