Ultrasmooth optical surfaces are of important application due to low-loss and low-scatter properties. Especially, in soft X- ray region, the surface roughness of substrates of multilayered mirrors is demanded to be below 1 nm rms. Furthermore, the excellent reflectors usually take advantage of the surface roughness less than 0.1 nm rms. Polishing by a tin plate is one of the remarkable methods for fabricating mirror surfaces. We studied the tin polisher for polishing ultrasmooth surfaces. The characteristic of tin as a polisher material is presented. Non-contact, direct-contact and selfweight polishing states are studied. Polishing experiments on many materials are carried out with perfect surface quality. Surface roughness around 0.5 nm rms is obtained on BK7 glass, Zerodur, and sapphire plates. This paper submits a profile of the work on superpolishing with a tin polisher.