An apparatus for measuring fuel consumption in an internal combustion engine the fuel supply system of which has a circulation line for the purpose of cooling the fuel pump. At the times when fuel consumption is measured, this circulation line is short-circuited downstream of a flow measuring device disposed in the supply line, so that then only the quantity of fuel delivered to the engine flows through the supply line from the tank, thus flowing through the flow measuring device as well. Because of the heating up of the fuel, the shortened fuel circuit is effected only at such time as a measurement procedure is desired. In the exemplary embodiment, a 3/2-way valve is used for switching over the fuel return line.