A lower floor panel (20) and an upper floor panel (22) which is having a sunken storage portion (20D) is provided in front of a seat supporting base (12) which supports an operator's seat (16) with in a cab (11). A storage space (24) is provided between a sunken storage portion (20D) in the lower floor panel (20) and a flat horizontal surface (22A) of the upper floor panel (22). This storage space (24) can be used for installation of an air-conditioning unit (25). This lo means that the air-conditioning unit (25) can be accommodated in a floor portion in front of the operator's seat (16), permitting to suppress the height of the cab (11) to an appropriate level. In addition, installation of the air-conditioning unit (25) can make assembling and maintenance work far easier.