The procedure of the invention acquires a maximum RB value, which is a largest value of an R signal component and a B signal component, on the assumption that the signal format of an input video signal is an RGB signal, and obtains an RGB probability corresponding to a degree of largeness of the maximum RB value. The procedure also acquires a minimum color difference value, which is a smallest value of a color difference signal component, on the assumption that the signal format of the input video signal is a Y/color difference signal, and obtains a color difference probability corresponding to a degree of smallness of the minimum color difference value. The procedure compares the RGB probability with the color difference probability and thereby determines whether the signal format of the input video signal is the RGB signal or the Y/color difference signal. This arrangement effectively enhances the accuracy of discrimination between the RGB signal and the Y/color difference signal.