To provide a reversible compression type video signal compressing method and decoding method capable of performing sufficient compression even without performing predictive coding and completely decoding the original video signal at the time of decoding.

In compressing the video signal constructed as a frame group obtained by arranging still picture frames time sequentially so as to be completely reproduced, a correlation operation between respective color planes of a frame, a correlation operation of planes of the same color with each other between adjacent frames time sequentially, and processing of a signal flat part with little fluctuation in pixel values of the same plane of each frame are sequentially performed. Especially, as the processing of the signal flat part, a rectangular area 4 in which the fluctuation of the pixel values is within the number of two bits or less is extracted as the signal flat part from each plane (b). The signal flat part is represented by an upper left point (x7, y7) of the rectangular area, a lower right point (x8, y8), a pixel average value in the rectangular area and a difference value from the pixel average value of each pixel being within two bits or less is encoded as signal flat part data (c).