To provide a method by which the contamination of a vacuum piping or the like by solution is prevented and defoaming is carried out in a short time with a simple apparatus and simple operation, where air bubbles in the high viscous solution having a viscosity of ≥30 Pa.s are defoamed with a mixing tank equipped with agitating blades.

In the method for vacuously defoaming the high viscosity solution having a viscosity of ≥30 Pa.s, which comprises repeatedly exhausting and stopping the exhausting while agitating the solution by using a mixing tank equipped with agitating blades driven by an agitation motor, an exhaust device for exhausting the gas in the mixing tank and a valve for stopping the exhausting, the required agitation motive energy per unit volume is lowered so that the rotation number of the agitation blades becomes ≤1/2 of that of the agitation blades driven by the initial required agitation motive energy when the surface of the solution becomes impossible to reach the vacuum piping even when the valve for stopping the exhausting is fully opened.