An intake air duct serves as a passage for supplying air to an engine, and includes a first intake air passage, a second intake air passage, an opening-and-closing valve, an
interlocking member and a coiled spring. The opening-and-closing valve is disposed swingably in the second intake air passage so as to open and close the second intake air passage. The
interlocking member is disposed outside the first intake air passage or the second intake air passage, and swings by
interlocking with the swinging of the opening-and-closing valve. The coiled spring is disposed swingably outside the first intake air passage or the second intake air passage, and has a swinging end, which is brought into contact with a surface of the interlocking member. The swinging end slides on a surface of the interlocking member as the swinging of the interlocking member. The second intake air passage is opened by the opening-and-closing valve when a first urging force, which acts onto the coiled spring while the interlocking member swings so as to
interlock with the opening-and-closing valve being swung in an opening direction by a negative pressure exerted in the second intake air passage, is larger than a second urging force, which is exerted by a spring elasticity of the coiled spring. The second intake air passage is closed when the first urging force is smaller than the second urging force.