Abstract: When recording records into the IC memory group of a magnetic disk subsystem the records being composed of a count area of a fixed length, key area of a variable length and a data area of a variable length, data of a special format which is judged by ECC check to be an abnormal data is written at the top of the count area in each record. When reading an object record from the IC memory group, it is judged by ECC check whether or not the read data is an abnormal one. When the read data is judged to be an abnormal one, the read data is compared with the data of special format, and when there is found a coincidence between them, it is recognized that the read data are at the top of the count area. When writing data, data which are judged by ECC check to be a normal are suffixed to data which are judged by ECC check to be abnormal. When the read data are judged by the ECC check to be abnormal and judged by a next ECC check to be normal, it is recognized that the read data are at the top of the count area.