The photochemical behavior of 4-dimethylamino-4'-cyanodiphenylacetylene (DCDPA) has been examined by ab initio complete active space self-consistent field (CASSCF) and its second-order multireference Moller-Plesset perturbation (MRMP2) methods. DCDPA is mainly excited into an internal charge-transfer (ICT) state where the charge transfers from the dimethylanilino group into the benzonitrile part. In S-1, there are two stable geometries. One is an ICT state where DCDPA is skeletally relaxed from the stable geometry in So upon electronic excitation. The other is a diradical state where DCDPA takes a trans-bent form and the acetylenic C=C triple bond turns to be an ethylenic C=C double bond, as already confirmed in the case of parent diphenylacetylene. On the way to the globally stable trans-bent form in S-1, a conical intersection between the charge-transfer and the diradical state is located. The photochemical behavior of DCDPA in polar solvent has been also examined by CASSCF combined with the polarized continuum model. Thereby, it was found that a twisted ICT (TICT) state where the dimethylanilino group is perpendicularly twisted against the remaining part is formed in polar solvent.