To provide a fluid flow measuring device capable of compensating a change of the performance with passage of time, and to provide a method for measuring MAF (Mass Air Flow).

The fluid flow measuring device includes: a main flow channel 5 and a sub-flow channel 6 formed by separating the inside of a flow channel 2 with a separating wall 4; hot wire flowmeters 3m, 3s disposed on the main flow channel 5 and the sub-flow channel 6; a calibration fluid introduction mechanism 7 which is connected to the sub-flow channel 6 and introduces a calibration fluid to the sub-flow channel 6 when a target fluid is not allowed to flow the flow channel 2; and a correction calculation part 8 for correcting the hot wire flowmeter 3s on the sub-flow channel 6 so that the measured mass flow of the calibration fluid in the sub-flow channel 6 measured by the hot wire flowmeter 3s becomes the accurate mass flow, and correcting the hot wire flowmeter 3m on the main flow channel 5 so that the total mass flow of the flow channel 2 estimated from the measured mass flow of the target flow in the main flow channel 5 and the total mass flow in the flow channel 2 estimated from the measured mass flow of the target fluid in the sub-flow channel 6 become the same.