A water-insoluble cutting fluid base oil or grinding fluid base oil for metalworking includes a fatty acid polyoxyalkylene alkyl ether represented by the following formula (I-A), wherein a hydroxyl value of the fatty acid polyoxyalkylene alkyl ether is less than or equal to 2.0 mgKOH/g. R1—CO-(OA)n-OR3  (I-A) (In the formula (I-A), R1 represents a linear or branched, saturated or unsaturated monovalent hydrocarbon group having 15 to 21 carbon atoms and R3 represents a linear, branched, or cyclic monovalent hydrocarbon group having 1 to 8 carbon atoms. In addition, A represents an alkylene group having 2 to 4 carbon atoms, OA represents alkylene oxide, and n represents 1 to 6 which is an average mole number of added alkylene oxide (AO)).