In a rotation control device for rotating a rotation motor at speed in accordance with a boom raising operation amount, at the time of a combined operation for simultaneously performing a rotation action and a boom raising action, a limitation value of rotation torque in accordance with the boom raising operation amount is determined by rotation torque limitation value setting means of a controller, the limitation value serving as a rotation torque limitation value is sent to rotation speed control means, and a command value of the rotation torque is torque-limited by rotation torque limiting means.