The density matrix renormalization group is a variational approximation method that maximizes the partition function - or minimize the ground state energy - of quantum lattice systems. The variational relation is expressed as Z = Tr rho greater than or equal to Tr (<(1) over tilde>rho), where rho is the density submatrix of the system, and (1) over tilde is a projection operator. In this report we apply the variational relation to two-dimensional (2D) classical lattice models, where the density submatrix rho is obtained as a product of the corner transfer matrices. The obtained renormalization group method for 2D classical lattice model, the corner transfer matrix renormalization group method, is applied to the q = 2 similar to 5 Potts models. With the help of the finite size scaling, critical exponents (q = 2, 3) and the latent heat (q = 5) are precisely obtained.