To provide a novel L-shaped side ditch construction method which brings about drastic cut of labor and costs necessary for modifying an existing U-shaped side ditch into an L-shaped side ditch.

According to the L-shaped side ditch construction method, an upper portion of a U-shaped side ditch forming block 1 embedded in the ground a is cut and removed to obtain a remaining block body 1A having a U-shaped cross section, and a block accommodating space s is secured on an upper portion of the remaining block body 1A, followed by arranging an L-shaped side ditch forming block 2 in the block accommodating space s in a manner being erected on right and left rising side walls 1a of the remaining block body 1A to block the remaining block body 1 from above. Then a hardening material 3 such as concrete and mortar is poured in the block accommodating space s, and then hardened, to thereby rigidly accommodate the L-shaped side ditch forming block 2 in the block accommodating space s on the upper portion of the remaining block body 1A. In this manner an area under the L-shaped side ditch forming block 2 and at the same time inside the remaining block body 1A serves as an L-shaped side ditch channel 4.