The present invention provides a water/glycol-based hydraulic fluid that includes a total fatty acid and dimer acid of more than 0.4 and no more than 1.2 mass % as a fatty acid lubricant, and also a phosphate ester at between 0.01 and 0.07 mass %. The phosphate ester has the following structure (1) here R1 and R2 each represent a hydrogen atom or a hydrogen group with a carbon number between 1 and 30, and may either be mutually identical or mutually different; R3 represents a hydrocarbon group with a carbon number between 1 and 20; R4 represents a hydrogen atom or a hydrocarbon group with a carbon number between 1 and 30; and X1, X2, X3, and X4 each indicate an oxygen atom or a sulfur atom, where these may either be mutually identical or mutually different.