A method for correcting a soft decision value includes: demodulating a reception signal, which is coded by an error correction code and modulated in order to represent one or multiple bits with one symbol; generating the soft decision value, which is used in decoding of the error correction code; multiplying the soft decision value by a weight factor to correct the soft decision value; estimating an amplitude of the reception signal in every predetermined measurement time period; and variably setting the weight factor in a symbol unit in accordance with an estimating result so that a corrected soft decision value approaches to a predetermined amplitude range.