PURPOSE: To improve the replaceability of the water in the main body section of a water storage tank provided with the tubular main body section having a diameter larger than the diameter of a pipeline in the middle of the pipeline.

CONSTITUTION: An inflow path 3 and an outflow path 5 are connected to one end side and the other end side of a main body section 2. One side and the other side in the main body section 2 is partitioned by an intra-tank water outflow plate 11 movable between one end side and the other end side in the main body section 2. The inflow path 3 and the outflow path 5 can be selectively connected to one side and the other side of the main body section 2. The intra-tank water outflow plate 11 is moved by the water current flowing into one side in the main body section 2 from the inflow path 3, thereby the water on the other side in the main body section 2 is sent to the outflow path 5. The water in the main body section 2 is replaced when the intra-tank water outflow plate 11 is reciprocated.