PURPOSE: To penetrate toners into recording paper and to fix the toners firmly by applying ultrasonic oscillations to the toner images transferred on the recording paper while the toners are held melted in a fixing device.

CONSTITUTION: The toner images transferred on recording paper 4 is heated (to about 120W130°C) by the irradiation with a xenon lamp 8 of a fixing device 7, by which the toner images are kept melted, and since oscillations are applied thereto in said state from the oscillating surface (a) of an ultrasonic oscillation generator 9, the toner images penetrate deeply into the paper fibers and are firmly fixed. Stripping of the toner images from printed documents during processing as in the prior art is obviated and the images are unerasable with erasers. As a result, the above-mentioned method is adaptable to printing of important documents such as certificates in the same way as in printing with liquid ink.