Provided are a glass for a window which glass is suitably fitted to a semiconductor package made of a plastic and a glass window for the semiconductor package which glass window has various functions, the glass (1) being for use as a window material for a semiconductor package made of a plastic and having an average linear expansion coefficient of 120.times.10.sup.-7/.degree. C. to 180.times.10.sup.-7/.degree. C. at a temperature of 100 to 300.degree. C., or (2) the glass having the above average linear expansion coefficient and having a U content of 5 ppb or less and a Th content of 5 ppb or less, and the glass window (a) being formed of the above glass for a window, (b) having the function of a lens and the above average linear expansion coefficient, or being formed of a glass (c) having the above average linear expansion coefficient and having a U content of 5 ppb or less and a Th content of 5 ppb or less and containing Cu and phosphorus oxide, the glass window being capable of exhibiting a transmittance of 50% at a wavelength of 630 nm or less in terms of a spectral transmittance at a wavelength of 400 to 700 nm when the glass has a thickness of 0.5 mm.