To provide a manufacturing method of biodegradable lubricating oil base oil having a good low temp fluidity, a low cloud point, and, moreover, good oxidation stability and lubricity.

This base oil is manufactured by an ester exchange reaction of a mixture of 5-35wt.% fat contg. not less than 60wt.% monoene fatty acid having not less than 16 carbons and not more than 12wt.% diene fatty acid in the component fatty acid, 30-60wt.% fat contg. not less than 20wt.% trans acid in the component fatty acid, and 15-45wt.% fat or middle chain fatty acid contg. not less than 80wt.% middle chain saturated fatty acid, or its lower alcohol ester in the component fatty acid.