An optical recording/reproducing apparatus capable of using disc cartridges of at least two different sizes and/or a naked disc, on only one tray. The optical recording/reproducing apparatus is also capable of recording and/or reproducing information on/from one of the discs mounted on the tray by automatically detecting the disc cartridges and/or the naked disc positioned on the tray. The optical recording/reproducing apparatus has a case, a tray mounted to the case capable of being received and withdrawn into/from the case, and selectively accommodating disc cartridges of at least two different sizes, a support to support the disc cartridges positioned on the tray according to the size of the disc cartridges, a detection unit to detect the sizes of the disc cartridges positioned on the tray and the state of the discs accommodated in the disc cartridges, a rotating unit to rotate the discs in the disc cartridges positioned on the tray, and a recording/reproducing unit to record information on and/or reproduce the information from the discs rotated by the rotating unit.