To provide a collecting flat perforated plate of which construction is easy, and the occurrence of the dead region and deviation of circulated hot water in a water tank can be eliminated.

A discharge opening 16 is opened substantially in the center of a top plate 11a of a flat perforated plate case 11, and a flat perforated plate mouthpiece 17 for discharge is provided on the discharge opening 16. Along the peripheral edge part of the top plate 11a, a centralized inlet 18a is opened and a flat perforated plate mouthpiece 19 for inflow is provided on the centralized inlet 18a. A supply and discharge communicating pipe 15 communicated with the discharge opening 16 is fixedly provided substantially in the center of the flat perforated plate case 11, and a centralized inflow area 18 communicated with the centralized inlet 18a is formed in the periphery of the supply and discharge communicating pipe 15. The side plates 11b, 11c, 11d of the flat perforated plate case 11 are provided with a water supply inlet 12, a circulating outlet 13, and a discharge outlet 14, the water supply inlet 12 is communicated with the supply and discharge communicating pipe 15, and the circulating outlet 13 and the discharge outlet 14 are faced to the centralized inflow area 18.