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The main astronomical source of r-process elements has not yet beenidentified. One plausible site is neutron star mergers (NSMs), but fromperspective of the Galactic chemical evolution, it has been pointed out thatNSMs cannot reproduce the observed r-process abundance distribution ofmetal-poor stars at [Fe/H] $< -3$. Recently, Tsujimoto & Shigeyama (2014)pointed out that NSM ejecta can spread into much larger volume than ejecta froma supernova. We re-examine the enrichment of r-process elements by NSMsconsidering this difference in propagation using the chemical evolution modelunder the hierarchical galaxy formation. The observed r-process enhanced starsaround [Fe/H] $\sim -3$ are reproduced if the star formation efficiency islower for low-mass galaxies under a realistic delay time distribution for NSMs.We show that a significant fraction of NSM ejecta escape from its hostproto-galaxy to pollute intergalactic matter and other proto-galaxies. Thepropagation of r-process elements over proto-galaxies changes the abundancedistribution at [Fe/H] $< -3$ and obtains distribution compatible withobservations of the Milky Way halo stars. In particular, the pre-enrichment ofintergalactic medium explains the observed scarcity of EMP stars without Ba andabundance distribution of r-process elements at [Fe/H] $\lesssim -3.5$.

Contribution of Neutron Star Mergers to the R-process Chemical Evolution in the Hierarchical Galaxy Formation