Reaction mechanisms of nitrene, one of the most famous biradicals, have been frequently studied, and many spectral data have been obtained so far. In the present study, the experimental IR spectra of triplet 8-amino-l-naphthylnitrene ((3)ANN), a triplet diimine biradical 1,8-dihydro-1,8-naphthalenediimine ((DND)-D-3), and 1,2-dihydrobenz[cd]indazole (DBI), which are produced in the UV photolysis of 1,8-diaminonaphthalene in an Ar matrix and identified by a combination method of IR spectroscopy and DFT quantum chemical calculations, are first reported. (3)ANN is found to change to DBI by hydrogen-atom migration with bond making between the two nitrogen atoms upon visible-light irradiation (lambda > 580 nm) with its backward reaction caused by 350 nm irradiation. In addition, (3)ANN isomerizes to (DND)-D-3 by 700 nm irradiation, while its backward reaction occurs upon 500 nm irradiation. The wavelength dependences of these photoisomerizations are explained in terms of their electronic transition energies estimated by time-dependent DFT calculations. It is concluded that the novel reversible photoisomerization system among (3)ANN, (DND)-D-3, and DBI is totally different from the well-known photoisomerization between phenylnitrene and a seven-membered cyclic compound.