  We investigate the origin of the most iron-poor stars including SMSS
J031300.36-670839.3 with [Fe/H] < -7.52. We compute the change of surface
metallicity of stars with the accretion of interstellar matter (ISM) after
their birth using the chemical evolution model within the framework of the
hierarchical galaxy formation. The predicted metallicity distribution function
agrees very well with that observed from extremely metal-poor stars. In
particular, the lowest metallicity tail is well reproduced by the Population
III stars whose surfaces are polluted with metals through ISM accretion. This
suggests that the origin of iron group elements is explained by ISM accretion
for the stars with [Fe/H]$\lesssim -5$. The present results give new insights
into the nature of the most metal-poor stars and the search for Population III
stars with pristine abundances.
