The present invention inhibits an attachment from being damaged or broken when a large transverse load is applied to the attachment as a result of a slewing operation of an upper slewing body. A slewing control device (8S) includes an attachment information acquisition unit (800A), an angular velocity setting unit (801) and a slewing control unit (802). The attachment information acquisition unit (800A) acquires attachment information for setting the maximum slewing angular velocity based on the transverse load acting on an attachment (10S). The angular velocity setting unit (801) sets the maximum slewing angular velocity of an upper slewing body (12) based on the attachment information. The slewing control unit (802) controls a slewing drive unit (7S) such that the slewing angular velocity of the upper slewing body (12) does not exceed the maximum slewing angular velocity set by the angular velocity setting unit (801).