The present invention is intended to provide a fuel consumption evaluation system which obtains quantitative data to determine whether actual driving has consumed fuel more efficiently or less efficiently than average driving and gives the driver and/or manager concrete advice for fuel-efficient driving based on the obtained data, and also provide a fuel consumption evaluation system which decides, based on obtained fuel efficiency data, whether driving is more fuel-efficient than average driving and sets a target and gives the driver and/or manager concrete advice for fuel-efficient driving based on the obtained data in consideration of change in vehicle total mass without any influence of slopes and traffic flows and particularly encourages and instructs, in real time, the driver to do fuel-efficient driving. The system comprises: engine speed measuring means (2) which measures engine speed of a truck (1); accelerator opening degree measuring means (3) which measures an accelerator opening degree (α); vehicle velocity measuring means (4) which measures vehicle velocity (V); fuel flow rate measuring means (5) which measures a fuel flow rate (Fw); and control means (10) which evaluates fuel consumption of the truck from measured engine speed (N), accelerator opening degree (α), vehicle velocity (V) and fuel flow rate (Fw).