PURPOSE: To excavate a large hole in a limited ground by a method wherein a rail assembly to movably support a back hoe is supported at a part of the peripheral edge of a spot to be excavated to the upper end parts of a plurality of vertically expanding long members.

CONSTITUTION: A plurality of long members 16 are driven in a ground at intervals along the peripheral edge of a spot to be excavated, the upper end parts thereof are intercoupled through auxiliary members 32 and 34, and the auxiliary members 32 and 34 are intercoupled through a maintaining means 22. Further, a plurality of sleepers 24 are installed at intervals one side of the periphery of the spot to be excavated, a rail 26 is fixed on the sleepers 24 to build a rail assembly 18. After a back hoe 20 is installed on the rail assembly 18, a part surrounded with the auxiliary members 32 and 34 and the maintaining member 22 is excavated. Thus, the weight and the reaction force of the back hoe 20 is exerted on a long member 16 of the rail assembly 18. Since a force exerted on a ground is low, collapse of a hole wall is prevented from occurring.