A diagnosis control valve unit, provided in a vent pipe extending from a carbon canister, includes an air cleaner casing, an air cleaner filter and a diagnosis control valve substantially contained in the casing. A valve element, a valve seat and valve driving components of the diagnosis control valve are disposed in an internal space of the casing and the filter. The diagnosis control valve is firmly attached to an end of the casing. A valve element and a valve seat of a check valve are optionally provided in the casing, firmly attached to the opposite end of the casing. A vent pipe may be connected to that opposite end of the casing, in place of the check valve. The diagnosis control valve, check valve, or vent may be detachably connected to the casing.