To provide a manufacturing method for a hollow material which easily further charges damping material without need of use of a large diameter extending die.

This method is the one for manufacturing the hollow material continuously formed in the longitudinal direction of two pieces of flat-state outer plate parts parallel separately arranged and plural partition wall parts partitioning a space between both outer plate parts. Firstly, the hollow material body 1 formed from the plate-like part 2 for forming the one side of the outer plate having a bending part A in at least a part of the width direction, and an opening part 6, at which the plate-like part 3 for forming the other side of outer plate having rib parts 4 for forming the partition wall parts does not exist in at least the faced position to the bending part A, is manufactured by extruding formation. Successively, the hollow material body 1 is extended as the flat-state in the bending part A. Then, after charging the damping material from the opening part 6, a cover member 10 is fitted to the opening part 6 in the fixed state by friction-stirring joining method, etc.