PURPOSE: To easily construct a road side ditch using precast concrete by a method wherein the major part of the upper side of the side ditch is fixed to improve safety and utilization of the upper side thereof, and unstableness and noise due to setting of a cover avoided and proper tractive force is provided even if the amount of ditch is small by providing an adjustable-slope double section having a decreased width at a lower water level, and the side ditch serves as an earth retaining wall in the roadbed condition of embankment.

CONSTITUTION: The lower part of a precast concrete portal side ditch is supported against side pressure and a precast concrete adjustable bottom frame 2 which has rising parts on both sides nearly touching the inner side of the portal side ditch is built beforehand to constitute an adjustable slope small section so that a concrete is placed into the form most suitable for necessary drainage slope and drainage, and thereafter a portal side ditch 1 is mounted in combination therewith to form a double-section road side ditch. The adjustable bottom frame uses, if required, an adjustable bottom frame 3 with a protruding piece on one side and is connected to the lower end on the earth pressure side of the portal side ditch, whereby the road side which also serves as L- shaped earth retaining wall is provided.