To provide an inexpensive block for a waterway capable of maximally widening the width of the waterway formed even in the same width, capable of improving workability, and capable of preventing the looseness of a cover.

This block 10 for the waterway has a side wall 11 arranged in substantially parallel by separating at a predetermined distance, and a top plate 12 for connecting mutual upper parts of the side wall 11. The side wall 11 has a projecting part 15 extending in the longitudinal direction on its inner wall surface and supporting the cover 13 by projecting to the waterway side, and a plurality of ribs 18 extending downward from the projecting part 15. An outer wall surface is formed as a substantially vertical flat surface. An opening 14 closable by the cover 13 is formed in a part unconnected by the top plate 12 of the left-right side walls 11.