Disclosed is a fluid quality adjustment device provided with: a purification means (9) which deionizes a machining fluid; an anticorrosion means (8) which exchanges ions in the machining fluid for anticorrosion ions; a conductivity measurement means (10) which measures the actual measurement value of the conductivity of the machining fluid; a storage means (11) which previously stores a first reference value that is the upper limit of the conductivity of the machining fluid at which machining is enabled, a second reference value that is the lower limit thereof, and a third reference value that is between the first reference value and the second reference value; and a control means (12, 13) which performs control such that on the basis of the reference values stored in the storage means (11) and the actual measurement value of the conductivity of the machining fluid measured by the conductivity measurement means (10), passes the machining fluid through only the purification means (9) when the conductivity of the machining fluid exceeds the first reference value, stops the passage of the machining fluid through the purification means (9) and passes the machining fluid through only the anticorrosion means (8) when the conductivity of the machining fluid is less than the second reference value, and stops the passage of the machining fluid through the anticorrosion means (8) when the conductivity of the machining fluid exceeds the third reference value.