Reaction kinetics during microwave H-field heating and electric furnace heating were investigated to verify the effect of rapid heating and the non-thermal microwave effect. This work is focused on the ferromagnetic material NiMn2O4, and the reduction reaction under rapid heating is investigated using an in situ analysis device. The results showed that the activation energy varied with the heating rate under microwave H-field irradiation, and it was the same as that in an electric furnace when the heating rate was the same. On the other hand, under microwave H-field irradiation, the reduction reaction began from a significantly low temperature compared with that of electric furnace heating. The results suggest that the transition theory during microwave irradiation should be expanded to include the effect of spin waves excited by a microwave H-field on magnetic materials.