We have succeeded in imaging each surface carbon atom on Mo2C(0001) by scanning tunneling microscopy operated at low tunneling resistances and found a novel selective etching reaction of the surface carbon atoms by oxygen gas exposed at room temperature. On a carbon terminated Mo2C(0001) surface with both (root 3 x root 3)R30 degrees-honeycomb and c(2 x 4)-zigzag row structures, the etching reaction selectively occurred on the c(2 x 4)-zigzag row structure, leading to exposure of the underlying (1 x 1) Mo layer. Energetically excited oxygen formed on the surface is probably responsible for this new reaction event.