An intake air control device including an intake passage provided with reed valves to prevent a back flow of intake air and exhaust gas. The intake passage has a predetermined length and a substantially uniform section, except for an enlarged portion close to an intake valve. A reed valve assembly is arranged in the enlarged portion and has side walls which divide an inside space of the enlarged portion into a first space surrounding the side walls, and a second space surrounded by the side walls, to have a section similar to the uniform section, and are mounted with reed valves to allow an intake air to flow only from upstream to the first space. When the current engine speed condition is a high engine speed condition, a closing unit for closing the second space downstream of the reed valve is opened, and a communicating unit for communicating the first space with the second space is interrupted.