To obtain a circulating water tank in which a speed distribution can be adjusted simply.

A conduit part 1 for measurement is installed in a part of a circulating conduit 5 in which a conductive fluid 15 is circulated. A pair of magnets 20, 21 which can generate a magnetic field 39 to a direction which obliquely crosses the flow direction 24 of the conductive fluid 14 are installed. In addition, a pair of electrodes 22, 23 which can make a current flow to a direction at right angles to the flow direction 24 of the conductive fluid 25 and to the direction of the magnetic field 39 are installed.