PURPOSE: To precisely measure a fuel injection rate even in a low pressure fuel injection condition, to make no error in injection rate measurement after a finish of injection, and to enable installation in an engine.

CONSTITUTION: An injection rate measuring device is provided with a bias flow generating unit 3, into which fuel A inside a fuel supplying pipe 12 in a fuel supplying unit 2 flows via a flow dividing pipe 60, and an injection rate measuring unit 5 measuring a fuel injection rate of an injector 1 on the basis of a differential flow rate between a flow rate, which is measured by means of a laser Doppler velocimeter 50, inside the fuel supplying pipe 12 and a bias flow rate, which is measured by means of a laser Doppler velocimeter 52, inside the flow dividing pipe 60.