A reserve tank structure for enhancing the degree of freedom in the layout of the filling portion of the reserve tank and the mounting bracket and the degree of freedom in the layout of parts around the reserve tank. A reserve tank structure includes a reserve hose for connecting a reserve tank to a radiator. The reserve tank includes a tank body portion for reserving a cooling water with a filling portion that extends upwardly from the tank body portion and includes a water supply port on an upper end thereof. A hose plug is connected to an end portion of the reserve hose. Mounting brackets support an intermediate portion of the reserve hose wherein the mounting bracket is mounted on the filling portion.