Abstract: A video format mode detector for detecting the format mode of a video signal transmitted from a video data output device such as personal computer, which includes a counter for counting a horizontal synchronous signal input during one cycle of a vertical synchronous signal separated from the video signal, generating positive counting data and negative counting data, a data holder for holding the output from the counter whenever the horizontal synchronous signal separated from the video signal is applied thereto, a data adder for adding up positive holding data and negative holding data sent from the data holder whenever the horizontal synchronous signal is applied thereto, generating data which indicates the number of horizontal lines corresponding to one cycle of the vertical synchronous signal, a data comparator for comparing the positive holding data and negative holding data with each other, outputting data having the smaller value, and a mode discrimination part for searching an ROM included therein, gen