A door drive device is disclosed, which comprises a guide rail for defining in the body of the vehicle a way along which the slide door moves; first and second pulleys arranged at spaced positions of the vehicle body; an electric drive unit for driving the first pulley; an endless belt put around the first and second pulleys with a slack thereof; a bracket for connecting a portion of the endless belt to the door; and a tensioning device for tensioning the endless belt. Due to provision of the slack of the belt, various advantages are obtained particularly when the door drive device of the invention is used in combination with other door controllers.