To provide a forced stirring machine inside a hot water tank permitting effective heat exchange.

A hot water introducing pipe 11 is constituted to be a double pipe comprising an outer pipe 12 and a core pipe 13. A first linear part 17 is formed by extending the outer pipe downwardly, and a first bend part 18 is formed at the tip of the first linear part. A second linear part 19 extended upwardly is formed at the tip of the first bend part, and a second bend part 20 is formed at the tip of the second linear part. An extension pipe part 21 is formed at the tip of the second bend part, and the length dimension of the extension pipe part is set two to three times longer than that of the second bend part. A suction opening 22 is bored on the pipe wall in the lower position of the first linear part of the outer pipe. The core pipe is arranged inside the outer pipe in a position from the slightly upstream position of the suction opening forming position of the first linear part to a portion near the center of the second linear part, securing a slight gap with the inner wall of the outer pipe. A reducer 23 and a nozzle 24 are mounted on the upstream side and downstream side of the core pipe, respectively.