The invention discloses a door opening/closing device. The door opening/closing device has a rail module 10 equipped with a guide rail 20 extending in the front-rear direction, a drive device 30 attached to the guide rail 20, and a support member 40 capable of moving in the direction in which the guide rail 20 extends due to a driving force imparted by the drive device 30, by being: connected to the other end of a closing cable 51 which has one end thereof connected to the drive device 30 and is wound around a first pulley 21 provided on one end of the guide rail 20; and connected to the other end of an opening cable 52 which has one end thereof connected to the drive device 30 and is wound around a second pulley 22 provided on the other end of the guide rail 20. Furthermore, the rail module 10 is configured in a manner such that the drive device 30 is fastened to and positioned on the exterior of a vehicle chassis 1 in a manner such that part of the drive device 30 enters the interior of the vehicle chassis 1 through an opening P1 in the vehicle chassis 1.