The [2+2] photocycloaddition of 2-pyridone and methylacrylate through conical intersections was studied using ab initio molecular orbital (MO) methods. For the cyclic addition of 2-pyridone and methylacrylate, fourteen parallel-type conical intersections were located, which is the same type as that found in ethylene dimers. CiLC (CI/LMO/CASSCF) analysis of the conical intersections indicated that the ratio of regions proceeding toward products and reactants is nearly the same for all parallel-type conical intersections. These intersections are energetically located below the excited state of 2-pyridone and methylacrylate, and the order of the energies of the parallel type conical intersections corresponds to the quantity of products obtained experimentally. The regioselectivity of the photocycloaddition was explained from the conformation of the parallel type conical intersections with polarization.