Stability of three-fold lattice distortion in the insulating state of (DCNQI)(2)Cu is studied by exactly diagonalizing a two-band Peierls Hubbard model on the 6 x 2 lattice. Self-doping is essentially important and caused by strong commensurability pinning, which are a consequence of moderate coupling of DCNQI pi electrons with lattice and strong correlation of Cu d electrons.