PURPOSE: To reduce manpower required for the constructing and installing work at site by dividing a building constituting a field electric room, a field control room and a cable room into a plurality of blocks and carrying the blocks into a site after finishing the installation of boards and wiring of cables.

CONSTITUTION: A room scheduled between the columns A, B and between the columns I-II-I of a building is partitioned into L-shape by matching the columns 2 around the room with the room. Associated outer walls 3, inner walls 4, floors 4 and partitions 6 are then assembled thus constituting a field electric room and control room 1 of independent blocks 7, 8. A cable room is disposed under the floor of the field electric room and control room 1 and then board 10 are installed followed by installation of necessary cable trays and wiring of cables between the boards. The blocks 7, 8 are formed independently by splitting the columns of building and transported to a site where they are set at the position of a predetermined column 2. This method reduces manpower required for installing the field electric room, control room and cable room while partially cutting the installation work of board and wiring work of cables.