Lubricating oil composition

A biodegradable lubricating oil composition biodegradable in a river or ocean comprising:

• 100 mass parts of (A) an ester compound described below;
• 0.05 to 1.5 mass parts of (B) an amine salt of an acidic phosphoric ester described below;
• and
• 0.01 to 0.50 mass parts of (C) a monoesterified compound described below, wherein:
• (A) said ester compound of trimethylolpropane, a straight-chain saturated fatty acid having a carbon number of 8 to 10 and adipic acid, said ester compound satisfying the relationship of TMPmol %:FAmol %:ADmol % of 20 to 40%:40 to 70%:5 to 25%, respectively, provided that TMPmol % represents a molar percentage of a component derived from trimethylolpropane, FAmol % represents a molar percentage of a component derived from said straight-chain saturated fatty acid having a carbon number of 8 to 10, and ADmol % represents a molar percentage of adipic acid;
• (B) said amine salt of said acidic phosphoric ester represented by the following formula (1):
• wherein n represents an integer of 1 or 2,
• R′ represents an alkyl group having a carbon number of 4 to 6, and
• R″ represents a hydrogen atom or an alkyl group having a carbon number of 11 to 14;
• and
• (C) said monoesterified compound of an alkane diol having a carbon number of 3 to 8 and of succinic acid having an alkyl group having a carbon number of 8 to 18 or an alkenyl group having a carbon number of 8 to 18.