A rotor turning device for a wind turbine generator which can be used without having to modify the existing wind turbine generator. A rotor turning device includes, on the inner periphery, a first member acting as a fixing member for fixing to a coupling hub, and on the outer periphery, a second member acting as an operating member to which operational force for rotating a rotation axis of the wind turbine generator. A pair of the first members embracingly fixes to the coupling hub of a coupling, while the second member attaches to the pair of first members to form a handle-like shape.