through the control valves 10 1 to 10 3 and 12 1 to 12 4 into the outlet of each of the operation control valve groups 10 and 12 then hydraulic fluid will pass through pressure detectors 22 and 23 connected to the outlets respectively into respective drain conduits 40 and 41 the above mentioned pressure detectors 22 and 23 includes for example as depicted in fig2 dynamic pressure detector bodies 22a and 23a having internal restrictors 22b and 23b respectively pressurized hydraulic fluid which has passed through the respective restrictors 22b and 23b will pass through drain ports 22c 23c into drain conduits 40 and 41 respectively formed opposite to the restrictors 22b and 23b respectively and between the restrictors and the drain ports 22c and 23c are dynamic pressure detection or outlet ports 22d and 23d respectively which are arranged to take out pressurized hydraulic fluid as dynamic pressure the dynamic pressure taken out through the outlet port 22d is introduced as a pilot pressure through a conduit 26a into each one end of negative control valves 27 and 28 provided in the first and third variable displacement pumps 1 and 3 respectively whilst pressurized hydraulic fluid taken out through the dynamic pressure outlet port 23d is introduced as a pilot pressure through a conduit 29a into each one end of negative control valves 30 and 31 provided in the second and fourth variable displacement pumps 2 and 4 respectively further pressurized hydraulic fluid passing through the drain ports 22c and 23c of the pressure detectors 22 and 23 and the drain conduits 40 and 41 respectively is introduced as a pilot pressure through conduits 26b and 29b respectively into the other end of each of the above mentioned negative control valves 27.