A surface-treating agent comprising a perfluoro(poly)ether group-containing compound represented by the formula (A): Rf-(PFPE-Z 1 -Y-Z 2 ) n -R. In the formula; Rf represents an alkyl group having 1 to 16 carbon atoms and optionally substituted by one or more fluorine atoms; PFPE at each occurrence is independently a group represented by formula: - (OC 6 F 12 ) a -(OC 5 F 10 ) b -(OC 4 F 8 ) c -(OC 3 F 6 ) d -(OC 2 F 4 ) e -(OCF 2 ) f -; Z 1 and Z2 at each occurrence are independently a single bond or a divalent organic group; Y at each occurrence independently represents a group comprising at least one -O-; provided that at least one structure formed of two or more continuous oxygen atoms is present in formula (A); n is an integer of 1 to 400; R is a hydrogen atom, a fluorine atom, -CF 2 -Cl, -COF, -COOH, -PFPE-Rf' or Rf'; and Rf' has the same definition as that of Rf.