The invention provides a built-in support pillar for a fuel tank. The built-in support pillar for a fuel tank includes: a support pillar main body that is configured to be disposed inside a box-shapedfuel tank, and that joins together mutually facing internal walls of the fuel tank; an engaging portion for positioning that is formed on an outer circumferential surface of the support pillar main body; and a bracket that is mounted on the support pillar main body, the bracket including an engaged portion configured to be engaged with the engaging portion, and a fixing portion to which a built-in component is fixed.