1. A disk array device that controls duplicate writing to a cache memory carried out by a master controller and a slave controller, wherein the master controller is provided with a master I/O unit, a master cache memory and a master cache control unit, the slave controller is provided with a slave I/O unit, a slave cache memory, a buffer memory that is caused to function as the cache memory by altering memory attributes, and a slave cache control unit, the slave I/O unit stores write data in the buffer memory and transmits, to the master controller, a write request to which has been added identifying information indicating that the write data has been stored in the buffer memory, as well as the write data in the case of receiving the write request and the write data from a host device, the master cache control unit stores the write data in the master cache memory and transmits the write request to which the identifying information has been added to the slave controller in the case of receiving the write request to which the identifying information has been added, along with the write data, and the slave cache control unit alters the attributes of the buffer memory where the write data has been stored to cause the buffer memory to function as the cache memory in the case of receiving, from the master controller, the write request to which the identifying information has been added.