Provided is a high-temperature lubricating oil composition, which is resistant to hardening and sludge formation and displays minimal evaporation loss and superior thermal stability even under high-temperature open system working machinery conditions such as those found in a tenter, and to provide a lubricating oil composition that provides excellent lubrication without damaging members used within working machinery. The lubricating oil composition comprises a polyolester-based synthetic oil and a 12-72C fatty acid and/or a diphenylamine derivative containing an arylalkyl having a number-average molecular weight of >=400 but <=800.