Time-resolved resonance Raman and absorption spectra have revealed that three transients are involved in the photochromic reaction of 6-nitro-1’,3’,3’-trimethylspiro[2H-1-benzopyran-2,2’-indoline] in the ns - mu s time region. The firstly appearing transient is identified as the lowest excited triplet state T-1. The secondly appearing transient is assignable to the photomerocyanine generated from the T-1 state by the cleavage of the C(spiro)-O bond of the spiropyran ring, and the lastly appearing transient is considered to be a dimeric species of the photomerocyanine.