An automatically opening/closing apparatus for vehicle of rack-and-pinion type is reduced in size and weight. The automatically opening/closing apparatus for vehicle is automatically opened/closed, by transmitting rotation of an electric motor to a back door through a drive power transmitting mechanism comprising a pinion and a rack. The rack is supported linearly reciprocably by a slide mechanism comprising a slide block and a guide rail. The slide block is formed to be sufficiently short to the entire length of the rack and the guide rail is accordingly formed to be short. Further, the rack is provided with a groove portion, and a holding member contacting with a holding face is disposed in the groove portion. Then, an interval between the rack and the opinion is maintained by the holding member, whereby engagement of the pinion therewith is ensured.