A sliding door operating mechanism for a vehicle to provide access to the vehicle through an opening in a wall thereof, includes at least one sliding door and a reciprocating power mechanism and a transmission mechanism for transmitting movement of power to the door for performing opening, closing and locking movements thereof. A beam structure carries the door, the power mechanism and the transmission mechanism. The beam structure extends across and is accommodated within an upper portion of the opening so as to be slidingly movably driven by the power mechanism between a relatively retracted position within the opening, in which the door is located in a first plane in a closed position confirmed within the opening, and a relatively less retracted position, in which the door is located in a second plane exteriorly of the opening allowing movement of the door along the second plane.