PURPOSE: To reduce cost by forming a stepped terrace at a position lower than a road surface by digging a slope on a low land side, and disposing a road width widening block, capable of creating a surface continuous to the road surface and having a wider width of the top surface than the effective width of the terrace, on the terrace.

CONSTITUTION: A low land side is dug to form a nearly horizontal stepped terrace 2 at a required position lower than a road surface. A foundation 21 is formed on the terrace 2. A pedestal block 4 manufactured in advance as a precast concrete block is placed on the foundation 21. A block 5 similarly manufactured is placed on the block 4. A lower end of a support section 6 is bond to a footing section 4a of the block 4 via a binding structure 25a. A reinforcing outer edge section 23 is bound to the top surface of the block 4 via a binding structure 25b to make the top surface of the block 5 continuous to the road surface R. Thus the installation is simplified to reduce the cost.