Provided is a sliding door drive device enabling suppression of bulging against a body panel being a fixing target. The sliding door drive device includes a long guide frame, a first driven pulley and a second driven pulley being supported at both ends of the guide frame in a longitudinal direction, respectively, a belt being wrapped around the first driven pulley and the second driven pulley, and a belt drive part driving the belt. The guide frame includes a guide wall guiding the belt between the first driven pulley and the second driven pulley. The sliding door drive device further includes a sliding plate being placed at a position sandwiching the belt with an inner side of a curved part of the guide wall, and sliding on the driven belt.