PURPOSE: To provide a method for efficiently manufacturing an Al alloy-made seamless small high pressure gas vessel having excellent pressure proof strength.

CONSTITUTION: In the manufacturing method for Al alloy-made seamless high pressure vessel by forming an Al alloy extruded material to the prescribed shape of a bottomed cylindrical body 5 after executing solution treatment and successively, forming a gas discharging hole to the upper opening part of the bottomed cylindrical body 5 by a cold die forging method to make a raw vessel and applying ageing treatment to this raw vessel, the extrusion-spreading work is applied to the Al alloy extruded material, and after executing the solution treatment to this extrusion-spread material 1, an impact work is applied to form the bottomed cylindrical body 5. By this method, as the bottomed cylindrical body 5 is formed by the impact working method at a stroke, the productivity is improved and the drawing crack and the deterioration of mechanical property caused by repeat of an intermediate annealing can be avoided. Further, as the extrusion-spreading work and the solution treatment are applied to the Al extruded material, the material structure before the impact work is made to be uniform and fine and therefore, the impact workability is improved and the uneven thickness is reduced.