To make a turn at any radius or turning speed, allowing smooth switching of turns.

When a swash plate of a variable volume pump 25 of a turning HST is inclined in the normal rotation side, a turning HST motor 27 rotates normally so that a left crawler driving shaft 12L rotates in the same direction as a right crawler driving shaft 12R. In this case, since the number of revolutions of the left crawler driving shaft 12L is less than that of the right crawler driving shaft 12R, the left crawler travels slower than the right one. This is a mild turn in which the machine body gently turns to the left. When the swash plate of the variable volume pump 25 of the turning HST is inclined further, the number of revolutions of the turning HST motor 27 decreases in proportion to the inclined angle, until it becomes zero. At this phase, the machine body turning is shifted to a turn using the left crawler as a pivot, a brake turn. When the swash plate of the variable volume pump 25 of the turning HST is further inclined, the swash plate of the variable volume pump 25 of the turning HST is inclined to the reverse rotation side, the turning HST motor 27 rotates in reverse, and the number of revolution increases in proportion to the inclined angle. At this phase, the machine body turning is shifted to a turn in which left and right crawlers are turned in opposite directions, a spin turn.