A reference range identifying device acquires and records, at a predetermined time interval, a measuring result of a load and exhaust temperature of an engine. The device identifies, based on combinations of the load and exhaust temperature recorded at multiple points of time, for each load zone, for example a 95% confidence interval of a distribution of exhaust temperatures as a reference range. Subsequently, the device calculates an approximate curve for lower limits and upper limits of reference ranges identified for the load zones. The device identifies a range sandwiched by the calculated two approximate curves as a reference range that changes in accordance with a load. The device, upon detecting that a current load and exhaust temperature of the engine is not included in the reference range, notifies the detected fact to a user.