An operating device for rotating a winding roller of a window blind, includes an upright rotating shaft rotatable to cause rotation of the winding roller and having a thread segment, a sleeve shell movably surrounding the thread segment, a carrier disposed within the sleeve shell to be movable axially between pressed and non-pressed positions to hold a force transmitting member that is engaged with the thread segment, and a rotation preventing unit disposed between the sleeve shell and the carrier so as to transmit an axially manual force applied to the sleeve shell to move the carrier axially so as to rotate the rotating shaft through engagement of the force transmitting member with the thread segment in the pressed position, thereby winding up a window covering.