A compact turning device for a heavy object, comprising a turning arm joined to the heavy object and a drive device drivingly turning the turning arm. The drive device further comprises a motor incorporating a rotor shaft connected to the rotating input part of a planetary gear type speed reducer on a same axis (A). Also, the turning arm is installed by joining its first plane to the plane of the planetary gear type speed reducer forming the rotating output part and its second plane orthogonal to the first plane to the heavy object. The turning arm and the drive device are disposed within the width (D) of the heavy object in the axis (A) direction of the turning pivot of the turning arm. The planetary gear type speed reducer of the drive device may be disposed in two front and rear stages.