A spout assembly (1; 101; 201) comprising a spout (3; 103; 203) welded to a container body, and a cap (4; 104; 204) to be threadably engaged with the spout (3; 103; 203), wherein the cap (4; 104; 204) includes a top plate, a side wall connected to an outer peripheral edge of the top plate and having an inner peripheral surface on which internal threads are formed, and an inner ring circumferentially provided to an inner surface of the top plate; the inner ring has an outer peripheral surface including, sequentially from the top plate, a first region having a first outer diameter, and a second region having a second outer diameter whose outermost diameter is smaller than the first outer diameter; the spout (3; 103; 203) includes a side wall having an outer peripheral surface on which external threads are formed, the spout (3; 103; 203) being made of a material having rigidity that is lower than that of the material for the cap (4; 104; 204); and the side wall of the spout (3; 103; 203) has an inner peripheral surface including, sequentially from an upper end facing away from the container body, a first region having a first inner diameter, and a second region having a second inner diameter whose innermost diameter is smaller than the first inner diameter; characterized in that, in a state in which the cap (4; 104; 204) is threadably engaged with the spout (3; 103; 203), the first region of the inner ring is in close contact with the first region of the side wall of the spout (3; 103; 203) throughout the circumference, the second region of the inner ring is in close contact with second region of the side wall of the spout (3; 103; 203) throughout the circumference, a predetermined part of the inner peripheral surface of the side wall of the cap (4; 104; 204) is in close contact with a predetermined part of the outer peripheral surface of the side wall of the spout (3; 103; 203), and a space is formed between an area in which the first region of the inner ring is in close contact with the first region of the side wall of the spout (3; 103; 203) and an area in which the second region of the inner ring is in close contact with the second region of the side wall of the spout (3; 103; 203). the cap (4; 104; 204) includes a top plate, a side wall connected to an outer peripheral edge of the top plate and having an inner peripheral surface on which internal threads are formed, and an inner ring circumferentially provided to an inner surface of the top plate; the inner ring has an outer peripheral surface including, sequentially from the top plate, a first region having a first outer diameter, and a second region having a second outer diameter whose outermost diameter is smaller than the first outer diameter; the spout (3; 103; 203) includes a side wall having an outer peripheral surface on which external threads are formed, the spout (3; 103; 203) being made of a material having rigidity that is lower than that of the material for the cap (4; 104; 204); and the side wall of the spout (3; 103; 203) has an inner peripheral surface including, sequentially from an upper end facing away from the container body, a first region having a first inner diameter, and a second region having a second inner diameter whose innermost diameter is smaller than the first inner diameter; characterized in that, in a state in which the cap (4; 104; 204) is threadably engaged with the spout (3; 103; 203), the first region of the inner ring is in close contact with the first region of the side wall of the spout (3; 103; 203) throughout the circumference, the second region of the inner ring is in close contact with second region of the side wall of the spout (3; 103; 203) throughout the circumference, a predetermined part of the inner peripheral surface of the side wall of the cap (4; 104; 204) is in close contact with a predetermined part of the outer peripheral surface of the side wall of the spout (3; 103; 203), and a space is formed between an area in which the first region of the inner ring is in close contact with the first region of the side wall of the spout (3; 103; 203) and an area in which the second region of the inner ring is in close contact with the second region of the side wall of the spout (3; 103; 203). the first region of the inner ring is in close contact with the first region of the side wall of the spout (3; 103; 203) throughout the circumference, the second region of the inner ring is in close contact with second region of the side wall of the spout (3; 103; 203) throughout the circumference, a predetermined part of the inner peripheral surface of the side wall of the cap (4; 104; 204) is in close contact with a predetermined part of the outer peripheral surface of the side wall of the spout (3; 103; 203), and a space is formed between an area in which the first region of the inner ring is in close contact with the first region of the side wall of the spout (3; 103; 203) and an area in which the second region of the inner ring is in close contact with the second region of the side wall of the spout (3; 103; 203).