The potential energy surfaces for the electrocyclic reactions of butadiene at the ground and lowest excited states are calculated by CASSCF molecular orbital methods. The reaction paths for the formation of bicyclobutane via decay from the lowest excited singlet state of butadiene through a conical intersection are clarified. The difference between the reaction pathways producing cyclobutene and bicyclobutane is also explained from the concept of conservation of the dynamical momentum. The dynamical momentum on the excited-state surface from cis-butadiene favors the reaction pathway to cyclobutene rather than that to bicyclobutane.