We report the magnetic field effect on the cooperative photo-induced spin state transition process between low-spin (LS) and high-spin (HS) states in a spin-crossover complex. When the magnetic field is enhanced from 0.5 to 7 T, the transition temperature scarcely decreases only by about 0.2 K. In contrast photo-conversion efficiency from LS to HS state and the relaxation time of photo-converted HS state have been enhanced 7.1 and 1.5 times, respectively.