To enable a larger slewing angle of 180° or more.

A slewing body 1 can be swung to the right and the left at a slewing angle of 90° or more only by slewinging and driving either one of the cylinder device 10(15) while the slewing body 1 can be slewed to the right and the left at a slewing angle of 90° or more on the other side only by swinging and driving the other cylinder device 15(10). At that time, the residual cylinder devices 10 and 15 are allowed to follow in a non-drive state. Two cylinder devices 10 and 15 are used to change over and control the oil pressure circuit so that the slewing body 1 can be slewed at a large slewing angle of 180° or more (0≤slewing angle ≤360°).