A folding door (26) is provided on a cab box (14), and when the door (26) is in the closed position, door panels (27, 28) of the door and a left side face cover (9) of an upper rotation body (3) are substantially flush with each other relative to the vertical direction. Further, catches (34, 35) are arranged on a left rear side face section (14E) of the cab box (14), and strikers (32, 33) individually engaging with the catches (34, 35) are arranged on the door panels (27, 29) of the folding door (26). Since the two strikers (32, 33) are engaged with the catches (34, 35) at the open position of the folding door (26), rattling etc. of the door panels (27, 29) can be prevented and the folding door (26) can be stably held.