PURPOSE: To prevent generation of erosive burning by arranging an injection nozzle, which injects combustion gas generated in a combustion chamber toward the rear, in the intermediate part periphery of a rocket motor main unit in a position where a generated amount of combustion gas is almost equal in both front and rear side combustion parts.

CONSTITUTION: A rocket main unit 1 is formed of both front and rear cylinder units 2, 3 and an annular nozzle mounting member 4 for connecting these cylinder units. While the lower from the intermediate part of the rocket main unit 1, that is, the rear from a rear end part of the front part cylinder unit 2 is floating-fitted into an outer cylinder 10. Further a secondary combustion chamber 13 is formed between the rocket main unit 1 and the outer cylinder 10. Here in the periphery of the intermediate part, which is in the point end part of the secondary combustion chamber 13, in the rocket main unit 1, a plurality of injection nozzles 14 for injecting combustible gas are arranged in the peripheral direction with an equal pitch. Each injection nozzle 14 is arranged in a position where combustible gas, generated in a plurality of combustion chambers 7, 8 set so as to almost equalize a generated combustible gas amount, is simultaneously jetted.