To obtain a liquid having very low absorbance compared to a conventional perfluoropolyether in wavelength less than 250 nm.

The perfluoropolyether uses expressed by formula: X1-(CF2O)n(CF2CF2O)m(CF2CF2CF2O)p(CF2CF2CF2CF2O)Oq-X2 (I), substantially free from chlorine atom which is detected by 19F-NMR, as the liquid transmitting light at <250 nm, especially at 157 nm wavelength, in an optical system. In the formula, each of the repeating unit -CF2O-, -CF2CF2O-, -CF2CF2CF2O-, and -CF2CF2CF2CF2O- statistically locates along the chain and the X1 and X2 are the same and different and each a perfluoroalkyl chain end expressed by formula -(CF2)zCF3, (in the formula z is an integer of 0-3), wherein at least one of the n, m, p or q is not zero and the ratio of oxygen atom and carbon atom is more than 0.33.