An image signal processing device includes a reference signal generating portion, a determination portion, and an extraction portion. The reference signal generating portion generates and outputs a reference signal that sequentially transmits reference pulses shifted from horizontal synchronization pulses on a horizontal synchronization signal by a predetermined shift time. The determination portion determines whether the horizontal synchronization pulses are normal by comparing time differences between input of the horizontal synchronization pulses and input of the reference pulses with the shift time. The extraction portion extracts line image data points from an image signal in synchronization with the input of the horizontal synchronization pulses that have been determined as normal.