To provide a water-soluble working fluid which can be circulated in a large amount, is not foamed even in working under high-pressure oil supply, is transparent and odorless, and exhibits a good lubrication performance.

The working fluid essentially contains (A) a (poly)alkylene glycol alcohol ether represented by formula (1) and (B) a cationic polymer which comprises 2-100,000 repeating units derived from a monomer having a basic nitrogen atom or a tertiary ammonium salt or a salt of the monomer and has an average molecular weight of 300-10,000,000. In formula (1), R1is a 1-9C hydrocarbon group; R2is H or methyl; R3is H or a 1-9C hydrocarbon group; and m is an integer of 1 or higher.

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