[en] A new approach to the derivation of boundary conditions for the spherical harmonics method in Psub(N) approximation is considered. The approach is based in the Yvon-Mertence ideas employing expansion of the function in question into a binary series with respect to the Legendre polynomials with the expansion coefficients defined at corresponding semiintervals of the angular variable μ variations. In the case the boundary conditions for the expansion coefficients, are unambigously formulated. The boundary conditions for odd approximations of the spherical harmonics method are derived on the example of the one-speed neutron transport in a plane slab. As an example the boundary conditions for the P₃ approximation are explicitly put down