Photoexcitation effects on the spin states were investigated for a prototypical spin-crossover complex [Fe(ptz)6] (BF4)2 (ptz=1-propyltetrazole) by the pump and probe technique at 77K. We have found (I) superlinear increase of density nHs of the photoexcited high-spin molecules when the excitation power density P exceeded a critical value (=120mW/cm2) and (ii) optical hysteresis between the P-increasing and the P-decreasing runs. These characteristic features are well reproduced by the Monte Carlo simulation based on the Ising model, in which the spin state of the respective Fe2+ion is represented by the pseudospins.