To provide a flow measuring method with which a flow can be measured without enlarging the scale of a system by effectively utilizing liquid level control using a leak flow path.

A flow measuring system comprises a reactor container 10 filled with a coolant, an internal container 22 which is connected with the reactor container 10 via a coolant outlet flow passage 13 and filled with the same coolant, a pump 30 including a leak flow path which is installed within the internal container 22 and circulates the coolant with the reactor container 10, and a flowmeter which measures a leak flow rate of flowing in the leak flow path. Liquid levels of the reactor container 10 and the pump 30 are adjusted to be equal in rating operation, operation of the pump 30 is then started, and a main flow rate of the system is calculated from a measurement value of the leak flow rate.