We report on the nanometer-scale modification of a MoS2 surface by scanning tunneling microscopy (STM) with an electric field lower than that required for field evaporation by STM. It is known that a Pt-Ir STM tip dissolves H-2 gas into atomic hydrogen which is chemically active. We applied this phenomenon to STM modification to lower the electric field necessary for atom detachment. A Pt-Ir tip was used to dissolve the H-2 gas on the MoS2 surface. The gas-solid reaction enhanced the evaporation of the top-layer sulfur atoms, which were removed at a low electric field of about 2.4 V nm(-1). The present study shows that we can control STM modification well with the same feedback loop as that used for STM observation.