To provide a disk array controller which does not perform a degeneracy operation through a recoverable error of a disk unit and also controls so that input-output performance to/from a host device is not be reduced.

This device is provided with a failure detecting part 35 which detects an error when plural disk units 1-1 to 1-n write and read data, a write buffer 31 that writes data whose errors are detected by the part 35 among data from a host device 40 and which should be written in a disk unit in a write buffer 32, a read controlling part 33 that transfers data which are read from except a disk unit which becomes an error to a host device and also writes it to a buffer 34 when the part 35 detects an error at the time of reading data from disk units 1, and transfers data from the disk units and data of the write buffers to the host device when there is no error, and a controller 36 which checks data of the write buffers and read buffers and performs error restoration processing of the disk units during data writing and reading processing.