To provide a construction method for renewing an existing side ditch which forms an optimal channel shape for the situation of an existing side ditch laying site without removing the existing side ditch and prevents the accumulation of sediment and the like, considering that the amount and flow speed of water running through an existing side ditch varies depending on where the existing side ditch is installed and also the actual state of a construction site where the side ditch is installed.

A construction method for renewing an existing side ditch 1 that has an accumulation of sediment inside includes the steps of measuring the interior width and interior depth of the existing side ditch 1, selecting a pair of channel shape flexibly-changing members 4 of an optimum size for the measured existing side ditch out of channel shape flexibly-changing members 4 of various sizes in the light of the measurement result, removing the sediment in the side ditch 1, and attaching each of a pair of selected channel shape flexibly-changing members 4 onto both corners in the width direction of the existing side ditch 1 separately, by which a water channel that prevents the accumulation of sediment in the existing side ditch 1 is formed.