Flow rate measuring method for radioactive off gas in local exhaustion facility

In the inside of an exhaustion duct extending horizontally for flowing radioactive off-gas, a baffle plate having a size substantially covering the cross section of the duct is pivoted at the upper end thereof by means of a rotational shaft. The baffle plate is thus pivoted to the duct in a state where in can incline in the radial direction of the cross section of the duct around the rotational shaft. An indication pointer is secured to the upper end of the baffle plate penetrating a duct wall, and the pointer is tiltable together with the baffle plate around the rotational axis as a fulcrum. A display plate having gradients showing a relation between the inclination of the indication pointer and the flow rate in the duct is disposed at the back of the display pointer. When gas flows in the duct, buoyancy is caused to the baffle plate in accordance with the flow rate of the gas, whereby the baffle plate is inclined to display the flow rate corresponding to the indication pointer. (I.N.)