A rocket nozzle comprising: a dual-bell nozzle; and a gas introducing section (40A, 40B, 40C, 40D) configured to introduce gas into a space surrounded by said dual-bell nozzle, wherein combustion gas flows in said space, wherein said dual-bell nozzle includes: a first stage nozzle bell-shaped and surrounding an upstream portion of said space; and a second stage nozzle bell-shaped and surrounding a downstream portion of said space, wherein said dual-bell nozzle has an inflection point between said first stage nozzle and said second stage nozzle, wherein said gas introducing section comprises: a manifold provided along a circumference of said first stage nozzle on an upstream side from the inflection point, a first pipe provided to supply turbine exhaust gas to said manifold, the rocket nozzle further characterized by a valve, connected with another pipe branched from said first pipe to operate in response to a control signal indicating an altitude of a rocket having said rocket nozzle such that a part of said turbine exhaust gas flows through said another pipe and said valve; and a first gas inlet (45, 63) provided to a first inner wall surface of said first stage nozzle, wherein, when the altitude of said rocket is higher than a first predetermined altitude and is lower than a second predetermined altitude which is higher than the first predetermined altitude, said valve is closed such that said turbine exhaust gas is introduced into said space through said first gas inlet from said first pipe. a dual-bell nozzle; and a gas introducing section (40A, 40B, 40C, 40D) configured to introduce gas into a space surrounded by said dual-bell nozzle, wherein combustion gas flows in said space, wherein said dual-bell nozzle includes: a first stage nozzle bell-shaped and surrounding an upstream portion of said space; and a second stage nozzle bell-shaped and surrounding a downstream portion of said space, wherein said dual-bell nozzle has an inflection point between said first stage nozzle and said second stage nozzle, wherein said gas introducing section comprises: a manifold provided along a circumference of said first stage nozzle on an upstream side from the inflection point, a first pipe provided to supply turbine exhaust gas to said manifold, the rocket nozzle further characterized by a valve, connected with another pipe branched from said first pipe to operate in response to a control signal indicating an altitude of a rocket having said rocket nozzle such that a part of said turbine exhaust gas flows through said another pipe and said valve; and a first gas inlet (45, 63) provided to a first inner wall surface of said first stage nozzle, a first stage nozzle bell-shaped and surrounding an upstream portion of said space; and a second stage nozzle bell-shaped and surrounding a downstream portion of said space, wherein said dual-bell nozzle has an inflection point between said first stage nozzle and said second stage nozzle, wherein said gas introducing section comprises: a manifold provided along a circumference of said first stage nozzle on an upstream side from the inflection point, a first pipe provided to supply turbine exhaust gas to said manifold, the rocket nozzle further characterized by a valve, connected with another pipe branched from said first pipe to operate in response to a control signal indicating an altitude of a rocket having said rocket nozzle such that a part of said turbine exhaust gas flows through said another pipe and said valve; and a first gas inlet (45, 63) provided to a first inner wall surface of said first stage nozzle, a manifold provided along a circumference of said first stage nozzle on an upstream side from the inflection point, a first pipe provided to supply turbine exhaust gas to said manifold, the rocket nozzle further characterized by a valve, connected with another pipe branched from said first pipe to operate in response to a control signal indicating an altitude of a rocket having said rocket nozzle such that a part of said turbine exhaust gas flows through said another pipe and said valve; and a first gas inlet (45, 63) provided to a first inner wall surface of said first stage nozzle,