A determining unit determines which area in an SDRAM is the area in which image data is stored. According to the determination of the determining unit, a control unit activates a power-down mode only for an area in the SDRAM, the image data being stored in the area. In order to determine whether or not the data stored in the SDRAM is image data, a code is previously added to the relevant data. Whether or not the relevant data stored in the SDRAM is image data is determined by the determining unit using the added code. A minimum of only one bit does well as the code for determining whether or not the data stored in the SDRAM is image data. Instead of previously adding a code to relevant data, it is also possible, for the same purpose, to store a code in an internal register of the SDRAM, the code indicating contents of data stored in the SDRAM.