Multithreshold decoders (MTD) for the self-orthogonal codes (SOC) are considered. It is shown that applying of MTD for the well chosen codes provides almost optimum decoding. Noted that MTD using is difficult to provide a low decoding error probability at the a large noise level due to a small code distance of SOC. For reducing the error probability we propose a concatenated coding scheme formed by SOC and a nonredundant accumulate code. Simulation results show that these concatenated codes allow decrease BER decoding on four decimal orders at the almost optimum decoding area of the used SOC.