Cyclic dihydrogenpolysiloxanes, hydrogenpolysiloxanes of specific siloxane unit formulas etc., a process for their production using hydrolysis/condensation, a process for the production of silica type glass moldings with an optical transmittance of 90% to 100% in the vacuum-UV region to UV region and an optical transmittance of 98% to 100% in the visible region to near infrared region by curing said cyclic dihydrogensiloxanes or said hydrogenpolysiloxanes in a mold, said silica type glass moldings, optical elements made up of the silica type glass, a process for the production of optical elements having such a silica type glass film layer by coating an optical element with the hydrogenpolysiloxanes and curing them, and optical elements having such a silica type glass film layer.