Ab initio calculations are performed to elucidate the mechanism of the photoisomerization of azobenzene. We obtain the excitation energies of the S-1(n-->pi (*)), S-2(pi-->pi (*)), and S-3(n(2)-->pi (*2)) states by complete active space self-consistent field (CASSCF) and multireference single double configuration interaction (MRSDCI) calculations. Two-dimensional potential surfaces of the ground- and excited states are obtained at the CASSCF level in order to investigate the isomerization pathways. A conical intersection between the ground state and the S-1 state is found near the midpoint of the rotation pathway, and causes a radiationless transition. On the other hand, the S-2 state has local minima at the cis and trans structures, so that the isomerization proceeds at the S-2 surface following the deexcitation.