Abstract: When the received symbol is located (FIG. 4) within one of regions G, H, I, L, M, N, Q, R, and S, 2 bits are decided to be most reliable to obtain their soft decision values=0 or 7. As for the remaining 2 bits, a soft decision value=0 to 7 is decided by soft decision in the I- or Q-axis direction. When the received symbol is located within one of regions A, E, U, and Y, all the 4 bits are decided to obtain their soft decision values=0 or 7. When the received symbol is located with one of regions B, C, D, F, J, K, 0, P, T, V, W, and X, 3 bits are decided to be most reliable to obtain their soft decision values=0 or 7. As for the remaining 1 bit, a soft decision value=0 to 7 is obtained by soft decision in the I- or Q-axis direction. A soft decision method that can implement soft decision in multilevel (amplitude and/or phase) modulation and can fully exhibit the correction performance of maximum likelihood coding can be provided.