A read only type optical disc, and disc drive apparatus for reading/additionally writing data into the disc. The disc has a recording material (7) formed in all of the tracks (6) by which data can be additional­ly recorded on the disc (l). This optical disc has a plurality of sectors (S₀ - S n ) forming a data read-only area (4) and a write/read area (3). Each area has track sectors (S₀ - S n ) each having a sector identifier portion (ID) in which address information is recorded and a data field portion (DF) to record data. Data­field identification flags (F₁, F₂) indicative of the read-only area (4) and the write/read area (3) are recorded in the respective identifier portions. On reading or writing this optical disc, the data field identification flag is detected to identify (l6, 26, 28) the kind of related area i.e., read-only area or write/­read area. Thus, it is possible to avoid (l8 - 22; 28, 34) erroneous recording of data into the read-only area. Upon reading operation, waveform equalization and binary-clipping level of reproduction signals can be well controlled (26, 37, 39). The gain of the servo loop can be switched (53, 54). The track searching operation can be stably performed (58 - 63). The data reading and data writing into the data write/read area can be improved.