An instrument that directly measures a flow rate and air-fuel ratio of exhaust gas, and from the flow rate and air-fuel ratio of the exhaust gas, calculates fuel consumption measures the fuel consumption at high response speed and with high accuracy. The instrument is provided with: an ultrasonic flowmater 2 that measures a flow rate Q EX of exhaust gas flowing through an exhaust gas flow path R; and an arithmetic unit 4 that calculates fuel consumption Fe of an engine with use of the exhaust gas flow rate Q EX obtained by the ultrasonic flowmater 2 and an air-fuel ratio AFR obtained by an air-fuel ratio sensor 3 that measures the air-fuel ratio AFR of the exhaust gas flowing through the exhaust gas flow path R.
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