A storage unit is provided which is capable of preventing the occurrence of readout errors even if two or more errors arise within a code, and a storage unit checking method is offered which can improve the productivity. Write data produced by BCH-encoding and compacting data s1 inputted into a flash memory, capable of correcting two errors within a code in an encoder, is written in a cell array. Data read out from the cell array 3 is error-corrected and decoded in an error correction/encoder to generate output data. In the check to be made at manufacturing (or putting on the market), examination data written in advance is successively read out by one block so that the errors within each code is counted. If the number of errors assumes 1 or less, that code is subjected to error correction, whereas the block including a code whose error number is 2 or more is treated as a defective block. The acceptance or failure is determined depending upon, for example, whether the rate of the number of defective blocks to the total number of blocks is below or not less than 1%.