1. A video-signal processing method for measuring a noise level of an input video signal, the video-signal processing method comprising the steps of: receiving an input video signal; generating a reference video signal that is delayed relative to the input video signal; defining a plurality of regions in one field or frame of the input video signal; detecting an intra-field or intra-frame feature for each region that is defined in the input video signal, the intra-field or intra-frame feature representing a noise level in the input video signal within a field or frame; detecting an intra-field or intra-frame feature for a plurality of regions defined in the reference video signal, each region of the reference video signal corresponding to one region of the input video signal; checking the intra-field or intra-frame feature for each region of the input video signal against a noise-free-region threshold, and excluding regions of the input video signal of which the intra-field or intra-frame feature is less than the noise-free-region threshold from subjects of noise-level measurement; outputting a result of noise-level measurement by statistically processing the intra-field or intra-frame features of regions of the input video signal not excluded; generating an inter-field or inter-frame difference signal of the input video signal; detecting an intra-field or intra-frame feature for a plurality of regions defined in the difference signal, each region of the difference signal corresponding to one region of the input video signal; and checking the intra-field or intra-frame feature for each region in the difference signal against the noise-free-region threshold, and excluding each region of the difference signal of which the intra-field or intra-frame feature is less than the noise-free-region threshold from subjects of noise-level measurement, wherein the step of measuring a noise level outputs the noise-level measurement result by statistically processing the intra-field or intra-frame features of regions of the difference signal not excluded as well as the regions of the input video signal not excluded.