Transverse single spin asymmetry, $A_N$, of very forward (η > 6) $\pi^0$ production plays an important role to understand the production mechanism from the view points of particularly diffractive and non-diffractive interactions. However, it has never been measured in detail because non-zero $A_N$ of $\pi^0$ has been usually interpreted by perturbative QCD framework. Since larger $A_N$ was observed by more isolated $\pi^0$ than less isolated ones recently, the diffraction is emerging as a possible origin of the non-zero AN of $\pi^0$. To disentangle this point, we installed a new electromagnetic calorimeter in the 0-degree area of STAR experiment and measured very forward π0 over the transverse momentum ($p_T$) range of 0 < $p_T$ < 1 GeV/c in 510 GeV $p^↑ + p$ collisions.
We report the first measurement of the AN in very forward $\pi^0$ production and its preliminary result
as a function of $p_T$ recently released, which will provide more precise answer to the role of the
diffractive interaction in near future by combined analysis with other STAR detectors.