To improve the workability of maintenance work of the crushing rotor.

A by-pass conduit 45 connecting a pair of main conduits 36 and 37 together is disposed, and a direction control valve 46 is arranged in the by-pass conduit 45. The direction control valve 46 allows the by-pass conduit 45 to communicate with the main conduits 36 and 37 through a throttle 46B when the flow rate in the by-pass conduit 45 is below a set value and switches from the communication position (a) to the cutoff position (b) or (c) when the flow rate in the by-pass conduit 45 exceeds the set value. Owing to the configuration, a closed circuit consisting of a hydraulic motor 35, the main conduits 36 and 37 and the by-pass conduit 45 is formed when the flow rate in the by-pass conduit 45 falls below the set value e.g. on stopping the hydraulic pump 33, and the crushing rotor 18 can thus be rotated manually, leading to an improvement in the workability of maintenance of the crushing rotor 18 e.g. on replacement of the crushing bit 19.