An installation structure for a writing instrument includes an outer cylinder and a lead advancing mechanism, which is housed in the outer cylinder and has a sleeve provided with a protruding portion protruding in the outward direction, for advancing a lead by a predetermined amount. The lead advancing mechanism is installed in the outer cylinder so as to prevent the lead advancing mechanism from coming off (e.g., decoupling from) the outer cylinder by making the front end of the sleeve abut a step portion formed on the inner peripheral surface of the outer cylinder and fixing the protruding portion provided on the sleeve to a second step portion formed on the inner peripheral surface of the outer cylinder.