A combustor for a gas turbine, comprising: a first cylindrical body which is configured to hold a fuel nozzle extending in an axial line direction of a central axial line (Ac) of the combustor and through which air flows, in operation, 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, together with an outer peripheral surface of the first cylindrical body, an air introduction channel through which air is to be introduced such that the air reverses course at an upstream end of the first cylindrical body toward the downstream side, 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, wherein an inner peripheral surface of the fitting convex portion has an outside narrowing surface, a parallel surface, and a reversing surface sequentially formed above the inner peripheral surface of the fitting convex portion from the downstream side toward the upstream end of the first cylindrical body, wherein the outside narrowing surface is located on the radial outside of the air introduction channel and is formed to extend inward in a radial direction toward the upstream end of the first cylindrical body, wherein the reversing surface is a curved surface connected to the end portion of the parallel surface on the upstream side, wherein an inside narrowing surface located on the radial inside of the air introduction channel and extending outward in the radial direction toward the upstream end side of the first cylindrical body is formed on the outer peripheral surface of the first cylindrical body and is formed in a portion which faces the outside narrowing surface located on the radial outside of the air introduction channel from the radial direction, and wherein an end portion on the upstream side of the outside narrowing surface located on the radial outside of the air introduction channel is located closer to an upstream end of the first cylindrical body than an end portion on the upstream side of the inside narrowing surface located on the radial inside of the air introduction channel. a first cylindrical body which is configured to hold a fuel nozzle extending in an axial line direction of a central axial line (Ac) of the combustor and through which air flows, in operation, 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, together with an outer peripheral surface of the first cylindrical body, an air introduction channel through which air is to be introduced such that the air reverses course at an upstream end of the first cylindrical body toward the downstream side, 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, wherein an inner peripheral surface of the fitting convex portion has an outside narrowing surface, a parallel surface, and a reversing surface sequentially formed above the inner peripheral surface of the fitting convex portion from the downstream side toward the upstream end of the first cylindrical body, wherein the outside narrowing surface is located on the radial outside of the air introduction channel and is formed to extend inward in a radial direction toward the upstream end of the first cylindrical body, wherein the reversing surface is a curved surface connected to the end portion of the parallel surface on the upstream side, wherein an inside narrowing surface located on the radial inside of the air introduction channel and extending outward in the radial direction toward the upstream end side of the first cylindrical body is formed on the outer peripheral surface of the first cylindrical body and is formed in a portion which faces the outside narrowing surface located on the radial outside of the air introduction channel from the radial direction, and wherein an end portion on the upstream side of the outside narrowing surface located on the radial outside of the air introduction channel is located closer to an upstream end of the first cylindrical body than an end portion on the upstream side of the inside narrowing surface located on the radial inside of the air introduction channel.