Magnetic field effects (MFEs) observed for photoinduced hydrogen abstract reaction between benzophenone and thiophenol, generating phenylthiyl and benzophenone ketyl radicals, in an ionic liquid of N,N,N,-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)amide (TMPA TFSA) were analyzed by using the stochastic Liouville equation (SLE) with the cage model and the solvent separated radical pair model, respectively. The MFEs observed on the yield of escaped benzophenone ketyl radical in the range of 0 < B ≤ 2.0 T were explained by the transverse spin relaxation in the radical pair caused by the large anisotropy of the g-value and the slow rotation of phenylthiyl radical in the IL. The calculated MFE was dependent on a rotational correlation time of the radical as well as the mutual diffusion coefficient for the translational diffusion of the radicals. The SLE analysis revealed that the IL has at least two different viscosity regions, causing the nanoscale cage effect on the radical diffusion in the IL.