A use of a water-soluble metalworking oil agent provided by blending the following components A, B, C and D: at least one of a condensed fatty acid obtained by dehydration-condensing a ricinoleic acid, and a condensed fatty acid obtained by dehydration-condensing a monovalent carboxylic acid with an alcoholic hydroxyl group of a condensed fatty acid obtained by dehydration-condensing a ricinoleic acid; an ester compound provided by a dehydration condensate of a monovalent or multivalent alcohol and a monovalent carboxylic acid; an amine compound; wherein the amine compound is selected from at least one of monoethanolamine, monopropanolamine, monoisopropanolamine, 2-amino-1-butanol, 2-amino-2-methylpropanol, butylamine, pentylamine, hexylamine, cyclohexylamine, octylamine, laurylamine, stearylamine, oleylamine, benzylamine, diethylamine, diisopropylamine, dibutylamine, dipentylamine, dihexylamine, dicyclohexylamine, dioctylamine, dilaurylamine, distearylamine, dioleylamine, dibenzylamine, diethanolamine, piperazine, diisopropanolamine, stearylethanolamine, decylethanolamine, hexylpropanolamine, benzylethanolamine, phenylethanolamine, tolylpropanolamine, tributylamine, tripentylamine, trihexylamine, tricyclohexylamine, trioctylamine, trilaurylamine, tristearylamine, trioleylamine, tribenzylamine, methyldicyclo hexylamine, dioleylethanolamine, dilaurylpropanolamine, dioctylethanolamine, dibutylethanolamine, diethylethanolamine, dimethylethanolamine, dihexylpropanolamine, dibutylpropanolamine, oleyldiethanolamine, stearyldipropanolamine, lauryldiethanolamine, octyldipropanolamine, butyldiethanolamine, methyldiethanolamine, cyclohexyldiethanolamine, benzyldiethanolamine, phenyldiethanolamine, tolyldipropanolamine, xylyldiethanolamine, triethanolamine, tripropanolamine and triisopropanolamine; and water, wherein a blend ratio of the component A is 10 mass% or more of a total amount of the oil agent, a blend ratio of the component B is 5 mass% or more of the total amount of the oil agent, and a blend ratio of the component D is 5 to 75 mass% of a total amount of the oil agent, for end milling of a difficult-to-machine material which is one of titanium, a titanium alloy, a nickel alloy, a magnesium alloy, a niobium alloy, a tantalum alloy, a molybdenum alloy, a tungsten alloy, a stainless steel and a high-manganese steel. at least one of a condensed fatty acid obtained by dehydration-condensing a ricinoleic acid, and a condensed fatty acid obtained by dehydration-condensing a monovalent carboxylic acid with an alcoholic hydroxyl group of a condensed fatty acid obtained by dehydration-condensing a ricinoleic acid; an ester compound provided by a dehydration condensate of a monovalent or multivalent alcohol and a monovalent carboxylic acid; an amine compound; wherein the amine compound is selected from at least one of monoethanolamine, monopropanolamine, monoisopropanolamine, 2-amino-1-butanol, 2-amino-2-methylpropanol, butylamine, pentylamine, hexylamine, cyclohexylamine, octylamine, laurylamine, stearylamine, oleylamine, benzylamine, diethylamine, diisopropylamine, dibutylamine, dipentylamine, dihexylamine, dicyclohexylamine, dioctylamine, dilaurylamine, distearylamine, dioleylamine, dibenzylamine, diethanolamine, piperazine, diisopropanolamine, stearylethanolamine, decylethanolamine, hexylpropanolamine, benzylethanolamine, phenylethanolamine, tolylpropanolamine, tributylamine, tripentylamine, trihexylamine, tricyclohexylamine, trioctylamine, trilaurylamine, tristearylamine, trioleylamine, tribenzylamine, methyldicyclo hexylamine, dioleylethanolamine, dilaurylpropanolamine, dioctylethanolamine, dibutylethanolamine, diethylethanolamine, dimethylethanolamine, dihexylpropanolamine, dibutylpropanolamine, oleyldiethanolamine, stearyldipropanolamine, lauryldiethanolamine, octyldipropanolamine, butyldiethanolamine, methyldiethanolamine, cyclohexyldiethanolamine, benzyldiethanolamine, phenyldiethanolamine, tolyldipropanolamine, xylyldiethanolamine, triethanolamine, tripropanolamine and triisopropanolamine; and water, wherein a blend ratio of the component A is 10 mass% or more of a total amount of the oil agent, a blend ratio of the component B is 5 mass% or more of the total amount of the oil agent, and a blend ratio of the component D is 5 to 75 mass% of a total amount of the oil agent, for end milling of a difficult-to-machine material which is one of titanium, a titanium alloy, a nickel alloy, a magnesium alloy, a niobium alloy, a tantalum alloy, a molybdenum alloy, a tungsten alloy, a stainless steel and a high-manganese steel.