Brake mechanisms (33) that apply braking to rotations of left and right rotational shafts (25) each include a projecting part (40) that faces a partition wall (15A) of a brake case (15) and projects radially inward from an inner diameter side of a piston (37), an engaging member (41) that is arranged in the inner diameter side of the piston (37) in a state of being movably engaged with the projecting part (40) by a predetermined dimension in the left-right direction, and a pin member (42) that is provided between the partition wall (15A) of the brake case (15) and the engaging member (41). A frictional resistance between the pin member (42) and the partition wall (15A) is set to a value smaller than a pressing force of the piston (37) at the braking. The projecting part (40) is provided with an engaging member insertion groove (40A) that the pin member (42) penetrates in the left-right direction, and the engaging member (41) is movably engaged with the projecting part (40) in the left-right direction.