PURPOSE: To simplify the maintenance of a titled plant by draining the water accumulated in a water conduit between a water-letting wall and a lock gate for providing a water wheel pit for maintenance.

CONSTITUTION: A drain pit 14 is connected to a water conduit 13 provided between the upper stream side of a water-letting wall 12 and a lock gate 2. Consequently, when the lock gate 2 is pulled down to an operating position to partition a waterway 1 and a drain pump 16 is operated to drain the water accumulated in the water conduit 13, the water conduit 13 is turned into a waterless space to be usable for a water wheel pit. Since maintenance workers can enter the water wheel pit 13 even during the operation of a water wheel, the maintenance work can be greatly simplified.