To provide a work machine capable of efficiently regenerating the energy owned by hydraulic fluid during a swing deceleration. In the present invention, when a swing deceleration detection section 57a detects the state of a revolving upperstructure 102 being decelerated, when an operation judgment section 57c judges the state that a double-tilting pump/motor 14 is not supplying hydraulic fluid to any of a boom cylinder 1 and an arm cylinder 3, and when the number of double-tilting pumps/motors 14, 18 having supplied hydraulic fluid to a swing hydraulic motor 7 in the state before the swing deceleration detection section 57a detects the state of the revolving upperstructure 102 being decelerated is equal to one or more, a pump valve control section 57d controls changeover valves 43a, 45b, 45d, 49d to open and increases the displacements of the double-tilting pumps/motors 14, 18 to the side to make suction pressures become higher than discharge pressures of the double-tilting pumps/motors 14, 18 so that the double-tilting pumps/motors 14, 18 operate as motors.