An injector bracket 10 is provided inside an air cleaner 5. The injector bracket 10 comprises a rectifying wall 10a bulging into a main chamber 5b, and a stream-line guide wall 10b in opposition to an air current M having passed through an element 8. The injector bracket 10 functions as a partition and forms a separate chamber 10e partitioned from the main chamber 5b in the air cleaner 5. An upstream injector unit 7 is arranged in the separate chamber 10e. The rectifying wall 10a is in the form of a symmetric blade.