1. An image signal encoding apparatus, comprising: image inputting means for reading an image from an original and converting said image into data values comprising a plurality of color components including lightness and chromaticity components and outputting said data values; storing means for storing each of said data values; dividing means for dividing said data values into data values of a rectangular pixel block of lightness component consisting of m.times.n pixels and data values of rectangular pixel blocks of chromaticity components consisting of m.times.n pixels, where m and n are positive integers; image analysis means for generating an average data value separation block by calculating an average lightness component data value of m.times.n pixels in said pixel block of lightness component and average chromaticity component data values of m.times.n pixels in said pixel blocks of chromaticity components and obtaining the difference between said average lightness component data value and said average chromaticity component data values for performing waveform analysis in said pixel blocks based on a plurality of statistics of said average data value separation block including variance, histogram values and maximum and minimum values and for outputting waveform information and said average data values of m.times.n pixels of said pixel blocks of lightness component and chromaticity components; region determination means for determining whether said rectangular pixel block correspond to a color, monochrome, or background region in the original based on said average data values of m.times.n pixels of said pixel blocks of lightness component and chromaticity components and outputting a result of region determination; switching means for outputting said data values of said pixel block of lightness component during the region determination process by said region determination means, and for outputting said data values of said pixel blocks of lightness component and chromaticity components after the region determination process is completed; bi-level converting means for performing adaptive bi-level conversion of said data values of said pixel block of lightness component based on the result of said region determination and said waveform information; bi-level image encoding means for encoding data values of said pixel block of lightness component converted by adaptive bi-level conversion; region storing means for storing the result of said region determination as region information; color image encoding means for encoding said data values of the rectangular pixel blocks of chromaticity components consisting of m.times.n pixels determined to correspond to the color region by said region determination means based on said region information stored in said region storing means after said region determination process on the whole original is completed; and transmission control means for transmitting at least one of said data values encoded by said bi-level image encoding means and said data values encoded by said color image encoding means.