This invention provides a method for the preparation of insulating oil having good oxidation stability, electric characteristics and resistance to copper corrosion which comprises subjecting a distillate (stock) within a temperature range from 250.degree. to 400.degree. C. in terms of the boiling point at atmospheric pressure to solvent refining to a desulfurization from 30 to 75% by weight to give a raffinate, said distillate having been obtained either by distillation at atmospheric pressure of a paraffin- or mixture-base crude oil and/or by distillation under reduced pressure of a residual oil from the atmospheric pressure distillation, subjecting said raffinate to hydrogenating refining to a desulfurization from 40 to 90% by weight and subjecting the refined product to solvent dewaxing, and if necessary, subsequently subjecting the dewaxed product to clay treatment to a sulfur content of the final product from 0.1 to 0.35% by weight.