A bicycle frame includes a top tube provided with bulge-processed regions at its front and rear end portions, formed with bores having axes which intersect a longitudinal axis of the top tube, for accommodating therein the upper end portions of the head tube and the seat tube, respectively. The frame further includes a down tube provided with a bulge-processed region at its front end portion, formed with a bore having an axis which intersects a longitudinal axis of the down tube, for accommodating therein the lower end portion of the head tube, and optionally provided with another bulge-processed region at its rear end portion, formed with a bore having an axis which intersects the longitudinal axis of the down tube, for accommodating therein the lower end portion of the seat tube. The frame has an optimized distribution of effective section modulus in the longitudinal direction of the top and down tubes, to realize a markedly improved rigidity and mechanical strength.