A vehicle body frame of a motorcycle comprising: a head pipe which steerably supports a front fork which pivotally supports a front wheel and a steering handle; a pair of left and right main frames which extend rearwardly and downwardly from the head pipe; and a down frame which extends rearwardly and downwardly from the head pipe at an angle steeper than an angle at which the main frames extend rearwardly and downwardly, wherein the head pipe includes, as integral parts thereof, a pipe portion which extends in the vertical direction, and a bracket portion which is contiguously formed on a rear portion of the pipe portion in a state where front end portions of the main frames are jointed to the bracket portion and a front end portion of the down frame is joined to bracket portion below the front end portions of the main frames, the main frames are joined to the bracket portion in a state where a widthwise center line of the main frames in the vertical direction is directed toward an intermediate portion of the pipe portion in the vertical direction as viewed in a side view, and an upper surface extension line and a lower surface extension line which are on the extension in the frontward direction of an upper surface and a lower surface of the main frames pass between an upper end and a lower end of the pipe portion as viewed in a side view, a front end portion of the down frame which is inclined so as to be directed to the intermediate portion of the pipe portion in the vertical direction is joined to the bracket portion below the front end portions of the main frames as viewed in a side view, and a main frame mounting portion which joins the front end portions of the pair of left and right main frames thereto from both left and right sides, and a reinforcing portion which is integrally and contiguously formed with an upper portion of the pipe portion and is arranged above the main frame mounting portion, are integrally formed on the bracket portion, characterized in that the reinforcing portion is formed of a vertical rib having a triangular shape as viewed in a side view and a lateral rib which is contiguously formed with an upper portion of the vertical rib so as to extend in the lateral direction in a triangular shape having an apex at a front side thereof as viewed in a plan view, and further characterized in that the main frames are joined to the bracket portion in a state where the widthwise center line of the main frames in the vertical direction is directed toward a center portion of the pipe portion in the vertical direction as viewed in a side view. a head pipe which steerably supports a front fork which pivotally supports a front wheel and a steering handle; a pair of left and right main frames which extend rearwardly and downwardly from the head pipe; and a down frame which extends rearwardly and downwardly from the head pipe at an angle steeper than an angle at which the main frames extend rearwardly and downwardly, wherein the head pipe includes, as integral parts thereof, a pipe portion which extends in the vertical direction, and a bracket portion which is contiguously formed on a rear portion of the pipe portion in a state where front end portions of the main frames are jointed to the bracket portion and a front end portion of the down frame is joined to bracket portion below the front end portions of the main frames, the main frames are joined to the bracket portion in a state where a widthwise center line of the main frames in the vertical direction is directed toward an intermediate portion of the pipe portion in the vertical direction as viewed in a side view, and an upper surface extension line and a lower surface extension line which are on the extension in the frontward direction of an upper surface and a lower surface of the main frames pass between an upper end and a lower end of the pipe portion as viewed in a side view, a front end portion of the down frame which is inclined so as to be directed to the intermediate portion of the pipe portion in the vertical direction is joined to the bracket portion below the front end portions of the main frames as viewed in a side view, and a main frame mounting portion which joins the front end portions of the pair of left and right main frames thereto from both left and right sides, and a reinforcing portion which is integrally and contiguously formed with an upper portion of the pipe portion and is arranged above the main frame mounting portion, are integrally formed on the bracket portion, characterized in that the reinforcing portion is formed of a vertical rib having a triangular shape as viewed in a side view and a lateral rib which is contiguously formed with an upper portion of the vertical rib so as to extend in the lateral direction in a triangular shape having an apex at a front side thereof as viewed in a plan view, and further characterized in that the main frames are joined to the bracket portion in a state where the widthwise center line of the main frames in the vertical direction is directed toward a center portion of the pipe portion in the vertical direction as viewed in a side view.