The potential energy surfaces (PESs) for the S-1-S-0 relaxation of trans-diphenyldiphosphene (DPP) in the P=P rotation-restricted condition have been calculated by means of ab initio complete active space self-consistent field and its second-order perturbation methods. Two types of PESs for the S-1-S-0 relaxation under P=P rotation-restricted condition have been scanned, starting from the stable geometry in S-1 where the two phenyl groups are perpendicularly twisted against the P=P part. The nonsymmetric inversion route where one of the CPP parts takes a linear geometry is favorable. On the other hand, the symmetric inversion route where both the angle CPP angles increase simultaneously is energetically unfavorable. This is contrastive with the case of azobenzene (AZB) in the N=N rotation-restricted condition. It has been confirmed that the discussion for the photochemistry of DPP is also true for a realistic diphosphene protected by a bulky substituent. The electronic factors on the shapes of PESs for the nonsymmetric inversion and the symmetric inversion routes of DPP have been analyzed and compared with those of AZB.