An insulator is provided with an acceleration pump attachment on the lower side of an insulating section. An acceleration pump is attached to the acceleration pump attachment. A passage is provided on the lower side of the insulating section. A conduit is provided on a connecting surface which contacts an engine to intercommunicate between an insulator air passage and the passage. A connecting passage is provided on an upper portion of the acceleration pump to intercommunicate between the passage and a negative-pressure introducing chamber. Thus, the acceleration pump can be easily disposed in a space at the lower side of the insulator, which is conventionally a dead space. In addition, an outer pipe to intercommunicate between the insulator and the acceleration pump is not required, whereby space saving can be achieved and a structure of interconnection can be simplified.