PURPOSE: To provide the titled metal processing agent which exhibits excellent putrefaction resistance and processing performance and which comprises a salt of a condensate of an ethylene oxide adduct of a substituted phenol or substituted naphthol/formaldehyde condensate with a polybasic carboxylic acid (or anhydride).

CONSTITUTION: An ethylene oxide adduct (a) of a substituted phenol/ formaldehyde condensate of formula I (wherein R is H, 1W12 alkyl, alkylene or aryl, or a group of formula II; y is 1W2; x is 1W15; and mx + n is 5W100) or an ethylene oxide adduct of a substituted naphthol/formaldehyde condensate of formula III and 10W40wt% polybasic carboxylic acid (b) (or anhydride) (e.g., maleic anhydride) are heated at 50W200°C in the presence of a catalyst in a solvent under normal or reduced pressure for 1W4hr to effect partial esterification, followed by a reaction with a neutralizer, such as an inorg. alkali, an (alkanol)amine, etc., thereby obtaining a salt (A) of a condensation product of the components (a) and (b). The component (A) may be compounded with an extreme-pressure additive (B), a rust preventive, an antiseptic, a surfactant, etc. so as to have a component (A) concn. of 5W100wt%.