Electronic property of hydrogen adatoms on a carbide-modified W(110) surface, W(110)(15 x 3)R14 degrees-C, was investigated by means of angle-resolved photoelectron spectroscopy. The work function of the surface decreases anomalously with hydrogen exposure. The hydrogen adatom polarizes positively due to bond formation with surface carbon atoms, resulting in the work-function decrease by as large as 1.0 +/-0.1 eV at saturation.