A first piece forms a part of a surge tank. A second piece forms the surge tank together with the first piece. The first piece includes an introduction hole through which blow-by gas is introduced. The first piece includes a first flange fixed to the second piece. The second piece includes a second flange fixed to the first flange. A cover is located inside the surge tank. The cover defines, together with an inner surface of the surge tank, a gas passage extending from the introduction hole into the branch pipe. A part of the cover is sandwiched between the first flange and the second flange.