After the matrix method in crystal statistics was introduced by R. Kubo Kramers and Wannier, and others, Onsager, using the abstract algebraic method, got the exact solution in the case of plane square net. His rather complicated method was simplified by Nambu considerably. We, independenty, attained the simplification of Onsager's method and have applied to the honeycomb lattice. And the exact solution has been obtained. From this, by the so called dual transformation we can get the partition function for triangular net.