To provide a flow rate measuring device capable of quick processing, and having high durability and high measuring accuracy.

A diaphragm 33 is deformed uniformly and finely by compressed air supplied from an electropneumatic regulator 21, and a test fluid having a flow rate corresponding to deformation of the diaphragm 33 is supplied into a measuring passage. Thus, the flow rate of the test fluid supplied from a fluid supply device 30 into the measuring passage is adjusted precisely. Consequently, accuracy of a flow rate accuracy confirmation device 20 is improved, and leak measuring accuracy of a fuel injection valve by the flow rate measuring device is improved. When measuring a leak from the fuel injection valve, the diaphragm 33 is stopped on a position where a force received from the compressed air supplied from the electropneumatic regulator 21 is balanced with its own elastic deformation force. Thus, a volume change of the measuring passage resulting from movement of the diaphragm 33 is not generated. Consequently, when measuring a leak from the fuel injection valve 1, the flow rate accuracy confirmation device 20 is not required to be separated from the measuring passage.