Signal processing device and transmission device

A signal processing device includes: a memory; and a processor coupled to the memory and configured to: perform soft decision of a value of, among bit strings, a predetermined bit string encoded with a soft decision code from a symbol assigned to, according to each value of the bit strings, the bit strings having been subject to encoding of an outer code with a turbo product code and encoding of an inner code with the soft decision code; decode the predetermined bit string with the soft decision code on a basis of a result of the soft decision; individually perform, from the symbol, the soft decision of a value of each bit string other than the predetermined bit string among the bit strings; and decode the bit strings with the turbo product code on a basis of a result of the decoding and a result of the soft decision.