To suppress the variation of the rotation speed of a gyratory hydraulic motor resulting from the influence of the actuation or the like of a construction machine and stabilize the gyratory motion of the upper gyratory body.

When the divided flow rate of the hydraulic oil supplied from a hydraulic pump 15 to a hydraulic motor 41 is reduced, by tilting an operation lever 32 for the gyratory motion and also tilting (composite operation) an operation lever 33 for the arm while rotating the hydraulic motor 41 for the gyratory motion, a drive signal is output to the solenoid 49A of an electromagnetic proportional valve 49 and the valve 49 is changed over from the lower pressure position (a) to the higher pressure position (b) in accordance with the decreased flow rate. And the variable capacity control part 42 of the hydraulic motor 41 is inclined to the little tilting side as shown with an arrow mark B by a tilting control cylinder 43. The tilting angle of the variable control valve 42 of the capacity is reduced in accordance with the divided flow rate, to increase the rotation speed of the hydraulic motor 41 and prevent the variation of the rotation speed of the hydraulic motor 41, resulting from the influence of the actuation of the arm cylinder 12.