PURPOSE: To enable to shorten a term of works, by a method wherein, in a subject building method for coupling together building in an atomic power plant, a burying trench is fabricated from blocks which are manufactured in a factory and have concavities and convexities in a vertical and a longitudinal direction.

CONSTITUTION: Levelling concrete 4 is placed in a bottom surface of a ditch 11, and a recess 12 is provided. A projection 10b on a lower block 10 for trench mates the recess 12 and the block 10 is placed thereat. The adjoining block 10 is coupled such that a projection 10a on an oening end engages a concave part 10d, and a connection is water-proofed by a sealing water-proof agent 13. The upper block 10 for trench is coupled such that the projection 10b engages a concavity 10 in the lower block for trench. After the upper and the lower trench is built, the ditch 11 is reburied by earth 14.