1. A method of matching stereo images, comprising the steps of: inputting first and second images IL and IR; developing said images IL and IR into a plurality of frequency component images FL1, FL2, FL3. .. , FLk, FLk+1,. .. , FLn and a plurality of frequency component images FR1, FR2, FR3,. .. , FRk, FRk+1,. .. , FRn, respectively; applying a secondary differential processing to each of said frequency component images; converting each frequency component image, after being applied the secondary differential processing, into ternary values pixel by pixel, thereby obtaining ternary-valued frequency component images TL1, TL2, TL3,. .. , TLk, TLk+1,. .. , TLn and ternary-valued frequency component images TR1, TR2, TR3. .. TRk, TRk+1,. .. , TRn; and performing a matching operation between said first and second images based on said ternary-valued frequency component images, wherein pixels in a one-dimensional window of the ternary-valued frequency component image TLk of said first image IL are compared in a one-to-one manner with pixels in a designated region of the ternary-valued frequency component image TRk of said second image IR, when said ternary-valued frequency component images TLk and TRk are identical in their frequency components, an evaluation raesult "P" is obtained when corresponding two pixels are both "p" or "m", while an evaluation result "Z" is obtained when the corresponding two pixels are both "z", and a similarity between two ternary-valued frequency component images TLk and TRk is evaluated by using the following equation: EQU Eall=.SIGMA..beta.k(PN)k+.SIGMA..gamma.k(ZN)k where PN represents a total number of pixels having the evaluation result "P", ZN represents a total number of pixels having the evaluation result "Z", and .beta.k and .gamma.k represent weighing factors.