PURPOSE: To widen space in a driver's cab without scaling up the whole cab box.

CONSTITUTION: A right-side overhang section 43, in which the inside is used as housing space 44, is mounted on the right-side wall section 35 of a cab box 31 and a rear-side overhang section 45, in which the inside is employed as housing space 46, on the rear-side wall section 33. Accordingly, a filter box 47 is housed in the housing space 44 of the right-side projecting section 43, an the rear side of a housing 48 is intruded into the housing space 46 of the rear-side overhang section 45, thus substantially zeroing the projection of the filter box 47 into a driver's cab 5, then largely reducing the projection of the housing 48 into the driver's cab 5.