To obtain a foamed sheet which can retain a sufficient surface wettability for a long time and is suitable for a self-adhesive tape substrate, etc., by subjecting an unfoamed olefin resin sheet contg. a blowing agent to a corona discharge treatment with a specified amt. of energy applied, then foaming the sheet, and subjecting the foamed sheet again to a corona discharge treatment with a specified amt. of energy applied.

The sheet is subjected to a corona discharge treatment with an applied energy of 500-10.000 J/m2 before the foaming and with an applied energy of 500-10,000 J/m2 after the foaming. The blowing agent, pref. a thermally decomposable one. is added pref. in an amt. of 0.1-30 pts.wt. to 100 pts.wt. olefin resin. Pref., the corona discharge treatment is carried out at 0°C or higher and not higher than the m.p. of the resin and in an inert atmosphere. The surface of the objective foamed sheet can be changed by embossing the sheet just after the foaming and before the surface is cooled.