To protect from debris flow and keep a forebay by flowing water by providing a structure such as an intake apron or the like on the low-flow river bed to intake surface water and providing the forebay where an intake valve is installed in a low-flow revetment on the water collision part side by a means for lowering the flow speed of a low-flow channel of a part where to intake.

A forebay 8 is formed in a water collision part of a low flow revetment 3 on the side where the force of water of a water flowing low flow channel 1 approaches when swift-stream river is normal, following the low flow river bed 2 in such a manner as to be equal to the low flow river bed 2 in the bed height, be wider on the low flow river bed 2 side, and have a trapezoidal plane shape by providing staircase-like groynes 9, 9a gradually decreased in width substantially at an angle of 45° in the direction of river width from the upstream and the downstream. An intake valve 13 is installed in the forebay 8 to take in water.