A drive device of an opening/closing body for a vehicle is obtained, said drive device being able to be stably assembled without requiring high positioning precision of a drum housing and a motor unit. One of the drum housing and the motor unit protrudes toward the other of the drum housing and the motor unit and has a plurality of boss parts, with which an anchoring member for anchoring the drumhousing and the motor unit is anchored. The other of the drum housing and the motor unit has a plurality of boss-receiving parts that receive the plurality of boss parts. At least one of the pluralityof boss-receiving parts has a tapered surface that expands in diameter toward the corresponding counterpart boss part. Clearance is provided between the boss part and the tapered surface while the drum housing and the motor unit are supported.