In order to investigate the electronic state of Me-3,5-DIP [Ni(dmit) 2]2, which has both localized spins and conducting electrons in the anion Ni(dmit)2 layers, we have measured the resistance in high magnetic fields. At low temperatures, we observe characteristic oscillatory behavior in the resistance as a function of the magnetic field direction. We also successfully detect Shubnikov-de Haas (SdH) oscillation with two frequencies. The results clearly show that the electronic state of this salt is highly two dimensional and there exists strong exchange interaction between the conduction electrons and the localized spins. The negative magnetoresistance is explained in terms of the suppression of the magnetic scattering.