A switch mechanism 70 discriminates whether a lever 40 is in a detent position or not. When the lever is in the detent position, a detent device 80 is activated by a fluid pressure to produce a force for holding the lever in the detent position. The operation of the detent device is dismissed when the lever is not in the detent position. A pressure sensor 97 detects whether the operator's hand is in touch with the grip 49 of the lever, thus determining whether the operation of the detent device 80 is necessary. An actuator 30 is operated or stopped in response to manipulation of the lever. When the lever is moved to a position other than the detent position, an operation reaction force of a level corresponding to the load pressure in the actuator is applied by reaction force device 60, 60' to the lever so as to enable the operator to sense any change in the load pressure and start of operation of the actuator. When the lever is in the detent position, the reaction force device 60, 60' does not operate so as to enhance the efficiency of the operation of the detent device 80 for holding the lever.