To execute data assurance while improving utilization efficiency of the area of a main storage memory without increasing the cost of a system, in an error judging circuit and the shared memory system.

An error judging circuit includes an error detection circuit unit which in SmEC-DmED using (k, k-3) Reed-Solomon code with P(x) as a primitive polynomial of m-order in a Galois field GF(2), detects if there is a one-block error, a two-block error or no error with respect to a polynomial expression I(x) of an original code as an object to be protected from an error with respect to data of m-bit block unit, and which detects the position p of the block error from S0p=S1 in the Galois extension field GF(2m).