In a borosilicate glass plate containing Li.sub.2 O, Na.sub.2 O and K.sub.2 O and for use as a window plate in an alumina package for containing a semiconductor device, Li.sup.+ and Na.sup.+ ions are partially ion-exchanged by K.sup.+ ions in a surface of the glass plate to form a surface layer having a compressive stress of 200 Kg.f/cm.sup.2 or more, in order to make a thin window glass plate for the package having a thickness of 0.7 mm or less and a sufficient mechanical strength. The glass also has a thermal expansion coefficient of 50-75.times.10.sup.-7 /.degree. C. over a temperature range of 30.degree.-380.degree. C. so as to be thermally compatible with the alumina package. The glass consists essentially of SiO.sub.2 55-64%, Al.sub.2 O.sub.3 7-20%, B.sub.2 O.sub.3 11-19%, Li.sub.2 O 0-8%, Na.sub.2 O 7-20%, K.sub.2 O 0-4%, and RO 0.5-7% on the base of weight percent, where R is at least one of Mg, Ca, Sr, and Ba. In order to insure the ion exchange, amounts of Li.sub.2 O, Na.sub.2 O and K.sub.2 O are adjusted so that (Li.sub.2 O+Na.sub.2 O/(Li.sub.2 O+Na.sub.2 O+K.sub.2 O) is 0.7 or more.