The present invention is configured so that it is possible to select any one of a first mode in which a first adjustment valve 13 and a second adjustment valve 20 operate solely on the basis of manipulation of a main manipulation tool (turning lever 8i), a second mode in which the first adjustment valve 13 operates solely on the basis of manipulation of the main manipulation tool (8i) and the second adjustment valve 20 operates solely on the basis of manipulation of a secondary manipulation tool (brake pedal 8j), and a third mode in which the first adjustment valve 13 operates solely on the basis of manipulation of the main manipulation tool (8i) and the second adjustment valve 20 operates on the basis of manipulation of the secondary manipulation tool (8j) while operating on the basis of manipulation of the main manipulation tool (8i).