A reinforcement (18) is attached to the right angle corner between the right rear side column material (10) and the right underside beam material (15) which form the cab (5) frame of a hydraulic shovel (1) so that buckling of the thin right underside beam material (15) can be prevented. The reinforcement (18) is made to penetrate through a penetrate portion (15d) formed on the upper side surface of the right underside beam material body (15a) so that the second bent surface portion (18b) formed on the reinforcement (18) is abutted against the lower side surface of the right underside beam material body (15a), and a thick floor plate (20) is abutted against the lower side surface of the right underside beam material body (15a). Consequently, when a load works in the direction for decreasing the angle between the right rear side column material (10) and the right underside beam material (15), the load is received by the floor plate (20) and even if the right underside beam material (15) is a thin hollow material having weak strength, the right underside beam material (15) is not buckled but protected thus achieving an increase in the strength of the cab (5). FIGS. 4A, 4B and 4C