Abstract: The graphic board can be mounted inside the expansion slot of the computer, and the frequencies of the horizontal synchronizing signal and the vertical synchronizing signal can be preset finely through presetting the values on the registers from the computer side.At the start of display, the display is carried out in synchronizing frequencies corresponding to the original 640 dots by 400 lines display of the CRT display device (step S100) Then, the synchronizing frequencies are increased gradually to the target values corresponding to the 640 dots by 480 lines display (steps S120 through S140). During this time, the CRT display device will continue synchronization against fluctuation of the frequency within an allowable range with the operation of an internal synchronization circuit. Consequently, the CRT display device can maintain synchronization all through to the target values, and thus a 640 dots by 480 lines display is available.