PURPOSE: To improve impregnation properties and voidless effect of varnish, by a method wherein a base material is passed in order through a low-viscosity liquid reserving sphere, hot siphon sphere and varnish reserving sphere and an impregnated low-viscosity liquid is evaporated by heating the base material in the hot siphon sphere.

CONSTITUTION: A base material 1 is immersed into a low-viscosity liquid 2a within a low- viscosity liquid reserving tank 2 through a base material supply source 1' and air within the base material 1 is replaced with the low-viscosity liquid 2a. Then the base material 1 is brought into a hot siphon chamber 4, heated by a heating roll 5 and the low-viscosity liquid 2a is removed through evaporation. In this instance, the air is removed beforehand by an exhaust pump 10 from within the hot siphon chamber 4 and the hot siphon chamber 4 is filled with saturated steam of the low viscosity liquid. Low-viscosity liquid steam 2' a generated by a base material heating part 5 is restored to the low-viscosity liquid reserving tank 2 from the hot siphon chamber 4 or restored to the same through the undersurface 4d of a flow from a condensed liquid pool 4c by being liquefied by a cooling coil 6. Therefore, the base material 1 is immersed into the varnish 2a within a varnish reserving tank 3 under a state where the base material 1 contains the saturated steam 2a and does not contain the air. The base material 1 is impregnated uniformly and sufficiently with the varnish 3a while the same is passing through a varnish reserving sphere 3.