In a CIRC encoder for carrying out error correction coding processing with a CIRC, a first interleaver and a second interleaver are provided which have different delay parameters, and these interleavers are switched in accordance with the recording density of an optical disc for recording data. Also, in a CIRC decoder, a first deinterleaver corresponding to the first interleaver and a second deinterleaver corresponding to the second interleaver are provided, and these deinterleavers are switched in accordance with the recording density of an optical disc for reproducing data. A recording/reproducing system is realized which is capable of carrying out appropriate recording and reproduction of data to and from an optical disc having a higher recording density can be carried out and is also capable of handling an optical disc of an existing format.