1. A signal discriminating device comprising: a comparing section comparing a motion magnitude between two fields of an input video signal with one or more predetermined motion magnitude patterns which are provided in correspondence with one or more signal sources, so as to determine one or more degrees of match or mismatch between the motion magnitude and the motion magnitude patterns; and a discriminating section discriminating a signal source of the input video signal in consideration of the degree of certainty or uncertainty of discrimination, the degree of certainty or uncertainty being determined on the basis of the degrees of match or mismatch determined in the comparing section.