PURPOSE: To enable to perform an adjustment over a wide flow rate range for an instrumental error which is needed as flow meter by constituting the flow meter such that a brake is exerted on rotation of a rotor corresponding to instrumental error characteristics which a positive displacement flow meter has.

CONSTITUTION: In case a flow meter has instrumental error characteristics not allowable within an instrumental error range necessary as a flow meter, an adjusting screw 25 is screwed in to compress a spring 24, and this causes a brake rod 23 to exert a friction force on a ring 20. As a result, a brake is exerted on a rotor 8 which is increased in a rotation resistance. Thus, in order to turn the rotor 8, a difference in pressure between a flow-in side and a flow- out side of a liquid to be measured increases, and the liquid to be measured leaking through a gap between a main body and the rotor 8 increases. This enables the instrumental error characteristics of the positive displacement flow meter to be brought to a instrumental error within a given range necessary as the flow meter over a wide flow rate range.