In this paper, we propose the use of a memory system which has a partially reliable scratch-pad memory (SPM). The reliable region of the SPM employing the ECC is higher soft error tolerant but larger energy consumption than the normal region. We propose an allocation method in order to optimize energy consumption while ensuring required reliability. An allocation method about instruction and data to proposed memory system is formulated as integer linear programming, where the solution archives optimal energy consumption and required reliability. Evaluation result shows that the proposed method is effective when overhead for error correction is large.