Method of deciding error rate and semiconductor integrated circuit device

An error rate determining method for a semiconductor memory circuit device having a primary memory circuit and a secondary memory circuit, wherein said primary memory circuit comprises dynamic memory cells, said error rate determining method comprising:

• performing a first operation in which when an information sustaining mode is set, a plurality of pieces of data are read from said primary memory circuit to generate inspection bits, and in which the generated inspection bits are stored in said secondary memory circuit;
• performing, in a fixed cycle, a second operation in which said pieces of data are read from said primary memory circuit and said inspection bits are read from said secondary memory circuit to detect whether an error exists in said pieces of data;
• adding a first predetermined value to a total value when an error is not detected in said second operation;
• subtracting a second predetermined value greater than said first predetermined value from said total value when an error is detected in said second operation;
• and
• adjusting said fixed cycle according to said total value to set an error rate.