To provide a coupling structure 1 of writing instrument members which can reduce compressive forces during the insertion of an inner cylinder into an outer cylinder even for airtight fit between the two members, can prevent the occurrence of any breakage or crack accompanied by a high load in a fitted area, and also can inhibit an airtightness loss accompanied by the reduction of fitting power over a long period of time.

In a coupling structure 1, an annular contact surface A21 for airtight and an annular rib A22 for getting over fit are formed back and forth in the outer cylinder while both an annular contact surface B31 contacting with the annular contact surface A21 and an annular rib B32 mating the annular rib A22 are formed outside the inner cylinder. Either of the annular ribs is divided into a plurality of arc rib pieces 5 by forming two or more notches 4. When considering that each apex of the rib pieces 5 along a virtual circle having the annular rib as its circumference forms one circular arc, the central angle of which ranges from 110° to 150° and the sum of those central angles range from 220° to 300°.