The analytical formulation of the neoclassical resonant diffusion is derived in order to investigate the ion radial transport in the non-axisymmetric mirror cell of the GAMMA10 tandem mirror, where GAMMA10 contains non-axisymmetric mirror cells for flute/interchange stability. The non-axisymmetric mirror, however, causes the neoclassical transport of ions. The remarkable point of the analysis is that the (ψ,θ¯,ℓ) coordinates are adopted in the derivation of the analytical expression. Especially one coordinate ℓ is taken along the drift orbit of resonant ions trapped in a magnetic mirror cell, by which the drift equation is able to be bounce-averaged along the 0th order ion orbit trapped in a non-axisymmetric mirror cell even in the case of large ion azimuthal drifts.