A lubricant for a vapor compression refrigerator using a lower hydrocarbon coolant which comprises, as a main component, a mineral oil having a kinetic viscosity at 40 DEG C of 3 to 150 mm /s, a pour point of -25 DEG C or lower, a viscosity index of 50 or more, a % CP and a % CA as determined by the n-d-M ring analysis of 50 or more and 12 or less respectively, a nitrogen content of 20 ppm or less, a sulfur content of 0.02 to 0.3 %, and an iodine value of 10g I2/100g or less; a hydraulic fluid composition for a vapor compression refrigerator comprising the lubricant and a lower hydrocarbon coolant; and a refrigeration unit employing the hydraulic fluid composition. The lubricant has excellent compatibility with a hydrocarbon coolant and is excellent in stability and lubricity, and hence is markedly useful as a lubricant for a refrigerator using a lower hydrocarbon coolant which is in no danger of ozone layer destruction or global warming.