5 includes a sealing portion 6 brought into contact with the outer peripheral surface of the locking member support 30 a an outer portion 5 a having protruding portions 5 a u2032 projected inward from two opposite lower ends thereof and an engagement member 10 interposed between the sealing portion 6 and the outer portion 5 a the sealing portion 6 the outer portion 5 a and the engagement member 10 have an approximate u shape in section the engagement member 10 has at both sides thereof button portions 14 projected outward through openings of the outer portion 5 a and pressing portions 12 extended downward from the button portions 14 to be located inside the outer portion 5 a in case the cover 5 is closed to cover the dust collection chamber 4 the protruding portions 5 a u2032 of the outer portion 5 a serve to urge the respective engagement protrusions 32 of the locking members 30 whereby the engagement members 30 are inwardly moved against a bias force of the compression spring 18 then the protruding portions 5 a u2032 at the outer portion 5 a of the cover 5 are moved downward beyond the engagement protrusions 32 to contact the stoppers 30 b located below the engagement protrusions 30 at that time the locking members.