A combustor includes: a combustion cylinder; and a combustion-chamber forming member disposed so as to be at least partially inserted into the combustion cylinder and forming a combustion chamber with the combustion cylinder. A radial-direction gap for introducing film air is formed between the combustion cylinder and the combustion-chamber forming member. A gas turbine includes a combustor; a compressor for generating compressed air; and a turbine configured to be rotary driven by combustion gas from the combustor.