An auto-correlation unit for pattern recognition to obtain auto-correlation functions as for sampled signals is disclosed N pieces of sample values X.sub.n (n=0 to N-1)are extracted from a series of the sample values expressed with an accuracy of optional multi-bits and the auto-correlation coefficients of these N pieces of the sample values obtained. The sum of sample values X.sub.n and X.sub.n-.tau. (.tau.=0 to P) are calculated. The squared value (X.sub.n +X.sub.n-.tau.).sup.2 of the added results is previously memorized in a ROM while being addressed by the resultant sum. The auto-correlation coefficients are available by feeding the output (X.sub.n +X.sub.n-.tau.).sup.2 of the ROM and executing calculation as defined by the following equation: ##EQU1##