To provide a spout and paper container for liquid, which is attached with the same free from such problems that an annular incision is broken when welded to the paper container, or a pouring cylinder of the spout and a cap are welded to each other.

The cap (20) is provided with an inner sealing ring (22), on the undersurface of its ceiling panel (21), whose outer peripheral surface comes into tight contact with the inner peripheral surface, in the vicinity of the end portion (141) of the spout (10), and is so constructed that a cap thread-engagement part (27) is formed on the inner peripheral surface of the cap peripheral wall, which is thread-engaged with a thread engagement part (13) provided on the outer peripheral surface of the pouring cylinder (14), while the vicinities of the end of the outer peripheral surface of the inner sealing ring is so formed as to be circumferentially equipped with a protrusion (26) gently projecting outward, and the wall thickness of the pouring cylinder section coming into close contact with the inner sealing ring is made equal to or thinner compared with the wall thickness of the inner sealing ring.