A water saving device comprising a plate member which is sandwiched within and maintained within a joint portion of a tap water line and divides the tap water line across the line of water flow. A main body block is bolted to one side of the plate member along the axis of and positioned in the middle of tap water line. The plate and the main body block each has a plurality of water saving holes arranged circumferentially so that the number of opposing holes in the plate and the main body block which are aligned to allow water flow therethrough can be changed by relative rotation of the plate member with the main body block around bolt 13.