The mechanism of the MO effect for the Mn50Sb44Pt6 film was discussed from the viewpoints of the structure and optical properties. As results, it was clarified; (1) the Mn50Sb44,Pt6 film after annealing at 300C was found to consist of MnSb with the ordered NiAs phase and PtMnSb with the disordered f.c.c. phase. (2) The resonancelike MO Kerr effect at about 2.1eV was the unique character only for the Mn50Sb44Pt6 film. (3) For the Mn50Sb44Pt6 film after annealed at 300C the maximum of θK at around 2.1eV, where xx was relatively small, was originated from ε′xy.