An optical transmission method, comprising: executing mapping processing so that information of one unit of each group of a plurality of groups, is mapped in a pattern of two or more Stokes vectors orthogonal between slots of a multi-time slot; generating an optical signal from an electric signal processed by the mapping processing, and transmitting the optical signal; receiving the optical signal and converting the received optical signal into an electric signal; and executing de-mapping processing for conversion into the information of one unit of each of a plurality of groups, by selecting high-likelihood bit information in association with the mapping processing in which the information is mapped in the pattern of Stokes vectors orthogonal between the slots of the multi-time slot, wherein, the executing mapping processing comprises mapping information of one unit, which is made up of three bits, belonging to one of two groups and information of one unit, which is made up of three bits, belonging to another of the two groups in the pattern of Stokes vectors orthogonal between two adjacent time slots and wherein, the executing mapping processing comprises mapping the information of two units in the pattern of the two orthogonal Stokes vectors by performing single-parity check coding on three bits that are information of one unit of a first group out of the two groups to generate a fourth bit for the first group, and by performing single-parity check coding on three bits that are information of one unit of a second group out of the two groups to use an inversion of a resultant bit as a fourth bit for the second group. executing mapping processing so that information of one unit of each group of a plurality of groups, is mapped in a pattern of two or more Stokes vectors orthogonal between slots of a multi-time slot; generating an optical signal from an electric signal processed by the mapping processing, and transmitting the optical signal; receiving the optical signal and converting the received optical signal into an electric signal; and executing de-mapping processing for conversion into the information of one unit of each of a plurality of groups, by selecting high-likelihood bit information in association with the mapping processing in which the information is mapped in the pattern of Stokes vectors orthogonal between the slots of the multi-time slot, wherein, the executing mapping processing comprises mapping information of one unit, which is made up of three bits, belonging to one of two groups and information of one unit, which is made up of three bits, belonging to another of the two groups in the pattern of Stokes vectors orthogonal between two adjacent time slots and wherein, the executing mapping processing comprises mapping the information of two units in the pattern of the two orthogonal Stokes vectors by performing single-parity check coding on three bits that are information of one unit of a first group out of the two groups to generate a fourth bit for the first group, and by performing single-parity check coding on three bits that are information of one unit of a second group out of the two groups to use an inversion of a resultant bit as a fourth bit for the second group.