PURPOSE: To improve the sight of a slope, facilitate the construction, prevent the breaking of the slope by the adhesion to irregularities of the slope, and realize the extension of a road or building site.

CONSTITUTION: A precast block 1 has lateral sides 2, 2 drawing a substantially quarter circular curve so as to be laterally separated by degrees upward from under, and a front surface substantially T-shaped as the whole. An edge part 3 is formed, extending from the lower end and lateral side ends toward the back surface side. A plurality of such blocks are arranged on a retaining wall or natural ground so that a substantially semicircular arch form appears on the slope by the sides 2, 2 of the block 1. A hardener 12 is placed in the pocket space of the block 1 with the retaining wall or bedrock. The block 1 is fixed by an anchor driven into the ground. In construction method for extension, the block 1 is put on the pile driven downward into the slope, and set and fixed in the same manner.