In order to evaluate hydrogen abundance in the Earth's core, high-pressure melting experiments of FeHx were carried out. The experiments were designed to simulate the iron-water reaction in the Earth's magma ocean so that FeHx was synthesized in hydrous silicate melts. Hydrogen concentration in liquid FeHx was measured for the first time with a new technique. The hydrogen concentration in the liquid FeHx was 20% higher than the coexisting solid FeHx. The hydrogen concentration in the liquid FeHx rapidly increased with increasing temperature. The accreted iron to the proto-Earth should have accumulated and have been in equilibrium with hydrous silicate melt at the bottom of the magma ocean. Hydrogen concentration in the core estimated from the result of our experiments was up to H/Fe = 0.69, which may reconcile most of the density deficit in the present outer core.