In fundamental tests, we revealed that work power could be estimated based on measured values for engine speed and exhaust-gas temperature during the operation of an agricultural tractor. Based on these results, we developed a prototype which indicates the operating conditions to the operator to reduce the fuel consumption of agricultural tractors. This device employed five operating areas according to engine speed and work power and was designed to indicate changes in operating conditions (travel speed gear, PTO gear and engine speed) to ensure the tractor would operate within a region enabling high fuel efficiency and low exhaust-smoke density. The device was mounted on a 24 kW tractor, and indoor tests, rotary tillage tests, mole drainage tests, fertilizing tests and inter-row cultivating tests were all conducted. Consequently, by following the notification of the device, fuel consumption was reduced by approx. 15% at a power ratio of 55-65% (the ratio of work power to maximum PTO output), approx. 25-30% at a power ratio of 35-40% and approx. 35-45% at a power ratio of 20% with full throttle. Fuel consumption was also reduced by approx. 10% at a power ratio of 25-30% and approx. 10-25% at a power ratio of 10-15% with 50% throttle.