The phenyl torsional potentials of trans-diphenyldiphosphene (trans-phosphobenzene; t-DPP), which is an analogue of trans-azobenzene (t-AZB), have been examined by means of ab initio complete active space self-consistent field (CASSCF) calculations. Though the electronic structures of t-DPP are similar to those of t-AZB, the phenyl torsional potentials are different from each other. In So, the potential energy curve of t-DPP has double minima at nonplanar conformations with C-2 and C-i symmetries, while that of t-AZB has only minimum at a planar conformation with C-2h. In S-1, the phenyl torsion of t-AZB is impeded from a planar geometry more than that in So. On the other hand, the phenyl torsion of t-DPP is promoted so that the phenyl groups are perpendicularly twisted against the P=P double bond around the Franck-Condon region. Comments on the experimental findings of realistic diphosphenes protected by bulky substituents are also made.