PURPOSE: To mount a smaller sized engine by enabling an output to reduce of the engine.

CONSTITUTION: When a circuit pressure of work machine system hydraulic circuits 5, 6, 7-11 rises to reach a certain preset pressure or more, a direction control valve 12 is switched to connect a steering system hydraulic circuit 4-10 to a relief valve (relief valve of setting relief pressure lower than relief valve built in with orbit roll 2) 13, through this direction control valve 12, provided in the downstream thereof, and a relief pressure of the steering system hydraulic circuit is decreased to a preset pressure of this relief valve 13.