Hydraulic oil supplied from pumps (12),(13) to an attachment actuator (60) is controlled by attachment control valves (27),(28). Hydraulic oil supplied from pumps (12),(13) to other actuators (52),(54),(56),(58) is controlled by other control valves (21) through (26). Control conditions for operating the attachment actuator (60) alone and control conditions for operating the attachment actuator (60) simultaneously with any of the other actuators (52),(54),(56),(58) are discriminated by the pilot pressure sensors (32),(34),(36),(38). Control characteristics of the attachment control valves (27),(28) are controlled by a controller (43) based on the discriminated control conditions.