1. A noise reduction circuit comprising: a video data storage means for storing noise-reduced video data of a directly preceding predetermined period; a plurality of correlation-utilizing noise reduction means each for forming and outputting noise-reduced data by utilizing a correlation based on a difference between two sets of data, said two sets of data being, data of a first pixel of input video data being subjected to noise reduction, and data received from said video data storage means of a second pixel being shifted by a predetermined amount in a time direction and/or space direction with respect to said first pixel, each of said plurality of correlation-utilizing noise reduction means for forming and outputting a difference cause discriminating signal indicating whether a difference is due to noise or a valid change of image, said difference cause discriminating signal being formed based on a temporal change of the difference, and said plurality of correlation-utilizing noise reduction means having different combinations of shift amounts in said time direction and/or said space direction; a selecting/combining control signal production means for forming and outputting a selecting/combining control signal for determining final noise-reduced video data based on said difference cause discriminating signals from said plurality of correlation-utilizing noise reduction means; and a selecting/combining means for obtaining said final noise-reduced video data by selecting or combining noise-reduced data from said plurality of correlation-utilizing noise reduction means, based on said selecting/combining control signal.