To provide the structure of a pump for a water tank to increase an amount of flow water and a flow water pressure without increasing the size of a pump, prevent the occurrence of choking and accumulation due to dust and scale in a flow passage in a pump, and reduce the occurrence of a trouble.

A flow passage 23 vertically extending from an inlet 21 to an outlet 22 is arranged in a pump 20 for a water tank and a rotary containing part 23a formed in size slightly larger than those of an upper and lower parts is arranged at the central part thereof. A rotary body consisting of a cylindrical magnet 24 and an impeller located inside the magnet is contained in a rotary body containing part 23a. A support shaft 25a is vertically extended at the central part of an impeller 25 and rotatably supported by support frames 26 and 27. The rotary body is rotated by an electromagnetic coil body 28 formed at an outer peripheral part and through rotation of an impeller 25 along with the rotation of the rotary body, a flow water pressure in the direction of flow water in a flow passage 23 is generated.