PURPOSE: To specify the liquid level of liquid by installing straightening members which have many flow passage opening distributed uniformly in a tank for upstream flow meter calibration at right angles of the flow direction of the liquid in the tank.

CONSTITUTION: When a calibration test of the flow meter 7 is started by opening a by-pass valve and the liquid is made to fall freely from a head tank 23 to a reservoir tank, the liquid in the head tank 23 flows down through flow passage openings 35 of an upper straightening plate 31 and a lower straightening plate 33. At this time, the liquid in the head tank 23 flows at right angles to the surfaces of the upper and lower straightening plates 31 and 33 and the distribution of the liquid becomes uniform in the surface direction of the upper and lower straightening plates 31 and 33, i.e. at right angles to the flow. Consequently, the flow velocity is prevented from becoming irregular and a local vortex is also prevented from being generated.