PURPOSE: To obtain the title channel having improved properties to prevent organisms from attaching thereto, by applying a wax (material) on channel members, which may be coated with a coating material, in a layer of a specific thickness or more, and by smoothing the surfaces.

CONSTITUTION: (A) Members, e.g. concrete, for constituting a channel are coated with a coating material if necessary, and then with (B) a wax (material), e.g. paraffin wax, in a melt or emulsion state in a layer 50μ or more thick, and smoothed to give the desired channel.