Each of a plurality of regions of a display unit is provided with an access flag indicative of whether a writing operation is performed on pixels in a corresponding region. It is determined whether the corresponding operation is performed on each of the regions based on the access flag. When the corresponding operation is necessary, image data written in a memory is compared with estimated image data. As a result, when a new writing operation is necessary, the new writing operation starts to be performed on each pixel after a previous writing operation is terminated. When the writing operation is terminated on all the pixels included in the region, the state of the access flag corresponding to the relevant region is changed to a state in which the writing operation is not necessary.