The main purpose of this proposed experiment is to investigate the time scale of the dynamical spin equilibrium for the iron(III) site in (n-C2H5)4N[ZnIIFeIII(mto)3] (mto = C2O3S) under longitudinal field (LF) up to 50,000 Oe. We will investigate the time scale to the dynamical spin equilibrium of the iron(III) site in (n-C2H5)4N[ZnIIFeIII(mto)3] by means of ZF- and LF-muSR, and will obtain the depolarization rate as a function of LF up to 50,000 Oe. Then we will estimate the frequency of the relaxation between the HS and LS states by using the Redfield equation. Judging from these backgrounds for the physical properties of A[ZnIIFeIII(mto)3], the dynamics of rapid spin equilibrium for (n-C2H5)4N[ZnIIFeIII(mto)3] are an original and attractive target for μSR.