Recent discovery of galaxies emitting surprisingly strong Lyman continuum at redshift z≃3 with Subaru telescope is discussed. These galaxies are brighter at 900 Å in their rest-frame (i.e. hydrogen ionizing radiation) than at 1600 Å (non-ionizing radiation). Such an extreme spectrum, which we call Lyman limit 'bump', has been never expected by any spectral models of galaxies so far. Here, we show that the observed strength of the Lyman bump can be explained by a model spectrum of a galaxy which has matter-bounded nebulae photoionized by very massive (∼ 100 M⊙) and metal-free (so-called Population III) stars. This suggests that such a primordial stellar population still exists at z ≃ 3, rather late epoch of the cosmic evolution.