A parking gear mounting structure for an automatic transmission includes an input shaft which is supported by a transmission case in such a manner as to be prevented from moving radially with respect to an axis of the input shaft, an output shaft which is arranged coaxially with the input shaft and is also supported by the transmission case in such a manner as to be prevented from moving radially with respect to an axis of the output shaft, and a planetary gearset provided between the input shaft and the output shaft. The planetary gearset has a pinion carrier which is provided with a parking gear in a fixed relationship thereto. The parking gear is engageable with a parking gear locking member for locking the parking gear when the transmission is in parking. The pinion carrier is mounted onto the output shaft to rotate as a unit with the output shaft. A connecting member is provided for transmitting driving power from the input shaft to the planetary gearset. The connecting member is in a fixed relationship to the input shaft. A bearing is arranged between the pinion carrier and the connecting member such that the pinion carrier is radially supported by the connecting member through the bearing to prevent a radial displacement of the pinion carrier with respect to the axis of the output shaft.