To obtain a method of manufacturing a twin tank rail for two-wheeled vehicles by which the lightening of weight and the reduction of cost are easily done.

By downward bending the rear end part of a hollow base stock (41) with a bender roller machine (20), a pair of primary hollow base stocks (41-1) are formed, divided into two of the right and left and also two sets of divided dies (23) having forming surfaces (24) which are corresponding to the external shape of the tank rail are provided. By clamping each primary hollow base stock (41-1) respectively from the right and left directions with each divided die (23), a pair of secondary hollow base stocks (41-2) at least the front end parts of which are respectively bent in the reverse right and left directions are formed and also, by surrounding the outer peripheries of the secondary hollow base stocks (41-2) with the divided dies (23) and, in such a state, forcing liquid into each secondary hollow base stock (41-2), the peripheral walls are plastically deformed along the forming surfaces (24) of the divided dies (23).