A frame structure of a motorcycle includes a head pipe gusset forged integrally with a head pipe, and a front fitting recess to which a front wall and front-half parts of right and left side walls of a down pipe are fit is provided with the head pipe gusset. The structure also includes a forged stiffener pipe gusset, and a rear fitting recess to which a rear wall and rear-half parts of right and left side walls of the down pipe can be fit is provided with the stiffener pipe gusset. The right and left front-half parts of the down pipe are inclined in accordance with a draft angle of the front fitting recess, and the right and left rear-half parts are inclined in accordance with a draft angle of the rear fitting recess.