We have measured the transient absorption of a negative photochromic spiropyran derivative, 3′,3′-dimethyl-6,8-dinitro-1′-octadecylspiro[(2H)- 1-benzopyran-2′,2 -indoline] by a nanosecond laser photolysis, in order to elucidate the dynamical behavior of the negative photochromic reactions. On visible light photolysis, the intensity of the absorption bands of the merocyanine (MC) form decreases and that of the spiropyran (SP) form increases. The transient absorption of the MC form consists of three bleaching components with the time constants of ≤25 ns, 240 ns and 3.4 μs. On the other hand, the transient absorption of the SP form has two rising components with the time constants of 620 ns and 4.0 μs. Based on the analogy to the photochromic reaction of the positive photochromic spiropyrans, the reaction pathway from the MC to SP form of the negative photochromic spiropyrans is discussed.