A seat support platform (17) is provided on a floor panel (13) of a floor member (12) of a cab (11). An air conditioner unit (25) is installed internally of the seat support platform (17). An under floor duct (27) is provided on the lower side surface (13C) of the floor panel (13) to form an external air induction passage (30) to guide external air from outside of the cab (11) toward an external air intake port (25H) of the air conditioner unit (25). Under floor duct (27) can be provided integrally with the floor member (12) without protrusion to the lower side, and the under floor duct (27) can supply cool external air into the cab (11). Even in the case of a small size hydraulic excavator, the under floor duct (27) can be provided integrally on the floor member (12) to secure broader space in the cab (11).