Observations with Subaru telescope have detected surprisingly strong Lyman continuum (LyC; ∼ 900 Å in the rest-frame) from some Lyman α emitters (LAEs) at z = 3.1. We have examined the stellar population which simultaneously accounts for the strength of the LyC and the spectral slope of non-ionizing ultraviolet of the LAEs. As a result, we have found that stellar populations with metallicity Z ≥ 1/50 Z⊙ can explain the observed LyC strength only with a very top-heavy initial mass function (IMF; 〈m〉 ∼ 50M⊙). However, the critical metallicity for such an IMF is expected to be much lower. A very young (∼ 1 Myr) and massive (∼ 100 M⊙) extremely metal-poor (Z≤5× 10-4Z⊙) or metal-free (so-called Population III) stellar population can also reproduce the observed LyC strength if the mass fraction of such 'primordial' stellar population is ∼ 1% in total stellar mass of the LAEs.