A fuel consumption measuring apparatus adapted to measure fuel consumption of an engine, the fuel consumption measuring apparatus comprising: a flow rate sensor that is provided in an exhaust gas flow path through which exhaust gas emitted from the engine flows, and adapted to measure a flow rate of the exhaust gas flowing through the exhaust gas flow path; an air-fuel ratio sensor provided in the exhaust gas flow path; and a calculation unit that is adapted to calculate the fuel consumption of the engine with use of the exhaust gas flow rate obtained by the flow rate sensor, an air-fuel ratio obtained by the air-fuel ratio sensor, and exhaust gas density, wherein the calculation unit comprises a fuel consumption calculation part that is adapted to, on a basis of the air-fuel ratio obtained by the air-fuel ratio sensor, change the exhaust gas density to calculate the fuel consumption, the exhaust gas density being used for the calculation of the fuel consumption; and a related data storage part that is adapted to store related data indicating a relationship between an air-fuel ratio and exhaust gas density, characterized in that, with use of the air-fuel ratio obtained by the air-fuel ratio sensor and the related data, the fuel consumption calculation part is adapted to determine the exhaust gas density at the air-fuel ratio to calculate the fuel consumption, wherein in the case of determining the air-fuel ratio obtained by the air-fuel ratio sensor to be in the rich region, the fuel consumption calculation part is adapted to determine the exhaust gas density at the air-fuel ratio, and in the case of determining the air-fuel ratio obtained by the air-fuel ratio sensor to be in the lean region, the fuel consumption calculation part is adapted to use a predetermined constant value as the exhaust gas density. a flow rate sensor that is provided in an exhaust gas flow path through which exhaust gas emitted from the engine flows, and adapted to measure a flow rate of the exhaust gas flowing through the exhaust gas flow path; an air-fuel ratio sensor provided in the exhaust gas flow path; and a calculation unit that is adapted to calculate the fuel consumption of the engine with use of the exhaust gas flow rate obtained by the flow rate sensor, an air-fuel ratio obtained by the air-fuel ratio sensor, and exhaust gas density, wherein the calculation unit comprises a fuel consumption calculation part that is adapted to, on a basis of the air-fuel ratio obtained by the air-fuel ratio sensor, change the exhaust gas density to calculate the fuel consumption, the exhaust gas density being used for the calculation of the fuel consumption; and a related data storage part that is adapted to store related data indicating a relationship between an air-fuel ratio and exhaust gas density, characterized in that, with use of the air-fuel ratio obtained by the air-fuel ratio sensor and the related data, the fuel consumption calculation part is adapted to determine the exhaust gas density at the air-fuel ratio to calculate the fuel consumption, wherein in the case of determining the air-fuel ratio obtained by the air-fuel ratio sensor to be in the rich region, the fuel consumption calculation part is adapted to determine the exhaust gas density at the air-fuel ratio, and in the case of determining the air-fuel ratio obtained by the air-fuel ratio sensor to be in the lean region, the fuel consumption calculation part is adapted to use a predetermined constant value as the exhaust gas density.