A method and an apparatus for building a thin lining on a tunnel to be excavated. The method includes the steps of boring a slot along an outer periphery of a cross section of the tunnel, filling lining material into the slot so as to form the lining material into the thin lining and removing bedrock surrounded by the thin lining so as to expose the thin lining. The apparatus includes a boring device for boring a slot in the tunnel and an injection device for injecting lining material into the slot. The boring device further includes a hollow frame, a plurality of rods fitted axially movably into the hollow frame, and a drive mechanism for rotating the rods and striking rear ends of the rods.