A writing instrument includes a front barrel and an ink tank. When, with a rear barrel for accommodating the ink tank removably fitted on the ink tank, a cap formed to cover the front barrel is fitted on the front barrel, the cap is positioned and fastened by abutment of the opening end of the cap. The cap has a hollow for accommodating a point assembly. The inside diameter of the hollow exceeds the outside diameter of the front barrel and the outside diameter of a rear end of the rear barrel. A barrel hold is formed along the circumference on the inner wall portion of the cap, so that the barrel hold grips the outer periphery of the rear end of the rear barrel. A guide portion is formed on the inner wall portion of the cap, closer to the front side than the barrel hold, to restrain over insertion of the rear end of the rear barrel into the cap. A writing instrument cap is assembled of a cap body and a clip by engaging an engaging portion formed on the cap body and a hooking portion formed in the clip, and is constructed so that engagement becomes gradually tight as the clip is slid along the axis from a tacking position.