Glucose-1-phosphate (G-1-P) is a biological product which, along with its derivatives, has various physiological applications. In order to develop fit efficient industrial process for G-I-P, enzymatic production of high concentration G-1-P was attempted by using phosphorylase purified From potatoes as an enzyme, several kinds of linear and branched dextrin differing in chain length and a mixture of KH2PO4 and K2HPO4 (Pi) as substrates. Since the maltotetraose is not active as a substrate for production of G-1-P, effective glucan concentration was defined on a basis of moles of glucose (C-eG) excluding if segment of maltotetraose from dextrin. Furthermore, since life reaction was a complicated reversible reaction involving many kinds of glucan differing in chain length. the inhibitory effect of a high concentration of substrate for the enzyme, an apparent equilibrium constant K-app was defined. K-app was expressed as function of initial concentration or glucan (C-eG,C-0) and phosphate (C-P,C-0). The concentration of G-1-P finally obtained at the equilibrium (C-G1P,C-eq) could he easily calculated from K-app regardless of chair) length and branch structures of glucan. As a result, a diagram expressing C-G1P,C-eq as a function of C-eG,C-0 and C-P,C-0 was proposed. A high concentration of G-1-P, namely , 230 mM G-1-P was experimentally obtained when C-eG,C-0 and C-P,C-0 were 516 and 3000 mM, respectively, and the enzymatic activity was 100 U.mL(-1).