PURPOSE: To effectively collect a foreign matter, such as dust generated at a drive part and a moving part as structure is simplified.

CONSTITUTION: A hollow air passage 22 is integrally formed in a base member 7 and a number of punched parts 23 for dust collection are formed in a base member 7 and a number of punched parts 23 communicated with an air passage 22 are punched in the upper surface part of the base member 7. Further, air passages 24 and 26 extended longitudinally of units 2 and 3, respectively, are respectively integrally formed at the tip parts 20 of side covers 15a and 15b and right and left end parts 21 of an upper cover 16. Slit-form opening parts 25 for dust collection communicated with the air passage 24 are respectively formed in the tip parts 20 of the side covers 15a and 15b and meanwhile, slit- form opening parts 27 for dust collection communicated with an air passage 26 are respectively formed in the right and left end parts 21 of the upper cover 16. A negative pressure for dust collection is fed in the air passages 22, 24, and 26.