An intake air routing device for an engine having a plurality of cylinders, includes a pair of half case bodies forming an air chamber, an air filter arranged in the air chamber and configured to purify outside air received via an intake duct, and a plurality of intake passages extending from the air chamber. The plurality of intake passages are connected with respective intakes ports of the engine. A partition plate is disposed in the air chamber, and is formed separately from the case bodies. The partition plate is arranged in the air chamber such that it extends in the air chamber by a predetermined length in a direction intersecting with a direction where the intake passages are lined up so as to partition the intake passages from each other. Such intake air routing device reduces intake interference while maintaining a small size of the intake air routing device.