Combustor, gas turbine

A combustor, comprising:

• a first cylindrical body which is configured to hold a fuel nozzle extending in an axial line direction and through which air flows toward a downstream side thereof;
• a second cylindrical body which is connected to the downstream side of the first cylindrical body;
• and
• an outer shell which has an inner peripheral surface configured to define an air introduction channel through which air is introduced such that the air reverses course at an upstream end of the first cylindrical body toward the downstream side together with an outer peripheral surface of the first cylindrical body,
• wherein the outer shell includes a fitting convex portion which is fitted to an inner peripheral side of a combustor insertion port provided in a turbine casing, and
• wherein an inner peripheral surface of the fitting convex portion has an outside narrowing surface which is formed to extend inward in a radial direction toward the upstream end of the first cylindrical body.