1. A noise reduction apparatus comprising: a frame memory for frame-delaying one of a video input signal and a video output signal from the noise reduction apparatus; a subtracter for calculating a difference between the video input signal and a frame delay signal read out from the frame memory; a comparison portion for comparing an output of the subtracter with an arbitrary threshold value; a first line memory for delaying a signal outputted from the comparison portion; a second line memory for further delaying the signal of the first line memory; and a block discrimination portion for calculating a sum of pixels inside a block constituted by each pixel of a video signal as a center and a plurality of pixels adjacent to the center pixel in both horizontal and vertical directions, and comparing the sum with an arbitrary threshold value, wherein the pixels constituting the block are acquired from the comparison portion, the first line memory and the second line memory.