1. An image display apparatus in which a display attribute of a specific area of the display can be modified based on an image signal input from an information processor, vertical and horizontal synchronization signals, and position information and display attribute information of the specific area, comprising: a resolution determination circuit which determines the resolution of the displayed image, based on the input vertical and horizontal synchronization signals, a dot clock generation circuit which generates dot clocks by multiplying the horizontal synchronization signal according to the determined resolution of the image, a horizontal display period detection circuit which detects a horizontal display period in one horizontal synchronization signal, using a signal in the image signal input from the information processor, dot clock counting circuit which actually counts the number of the dot clocks detected in the horizontal display period, a dot clock standard value reading circuit which reads a standard value of the number of the dot clocks in the horizontal display period in a line at the determined resolution of the displayed image, and a correction circuit which corrects the multiplication number in the dot clock generation circuit so that the actually counted dot clock number is identical to the standard value.