PURPOSE: To provide a hydrosonically microapertured thin thermo-plastic sheet composed of a thermoplastic material having nearly uniform-microapertures by ultrasonic vibrations, excellent in breathability, water permeability, etc., and useful as the starting, material for covers such as disposal diapers and feminine care articles and in filtering devices, etc.

CONSTITUTION: A desired thin sheet material has at least 1,000 microapertures per sq in. Further, it is preferred that the length of the end portion of said thin sheet material is increased by about 100% from that before the thin sheet material is microapertured; the average thickness is about 0.25 mil. or more; and each microaperture has an area of about 10 μm2 - about 100,000 μm2. Said thin sheet material is obtained by bringing a thin sheet material. into contact with the tip 54 of an ultrasonic horn so as to form an angle 56 (preferably, about 5-15 degrees) to a transport mechanism 22 and imparting thereto ultrasonic vibrations while filling a region 64 with a fluid 62 such as water to form microapertures.