A radiant tube includes a conduit; and one or more heat transfer promoters disposed in the conduit, wherein the heat transfer promoter includes a body part on a center side of the conduit, and protruding parts protruding from the body part toward an inner wall surface of the conduit, the protruding parts are on an outer periphery of the body part to be arranged in a circumferential direction of the conduit, the protruding parts includes first protruding parts having a distal end portion facing the inner wall surface across a gap L, and a second protruding part, the number of first protruding parts is greater than the number of second protruding parts, and a ratio (L/Dt) of the gap L to an equivalent diameter Dt of a conduit portion is x %, where the heat transfer promoter is disposed, and formula (1) is satisfied: 0.3%<x<7%.