There is disclosed a fluorooxyalkylene group-containing polymer composition represented by the following average composition formula (1). In the formula, the ratio of the Rf 2 group is 30 to 75 mole%, relative to a total of all of the terminal groups in the composition, and the fluorooxyalkylene group-containing polymer composition includes a one-terminal hydrolyzable silyl group-containing polymer and a both-terminal hydrolyzable silyl group-containing polymer, A-Rf 1 -B (1) wherein Rf 1 represents a divalent linear fluorooxyalkylene group, A and B represent groups represented by an Rf 2 group or a group represented by the formula (2), Z represents a divalent to heptavalent linking group having a silalkylene structure or a silarylene structure but no siloxane bond, X represents a hydrolyzable group, and Y is a linking group represented by C c H 2c or C 2 H 4 -C r F 2r -C 2 H 4 . As a result, there is provided a fluorooxyalkylene group-containing polymer composition of forming a coated film which is excellent in water- and oil-repellency, low dynamic friction coefficient, releasing property, property of wiping off stain and excellent in abrasion resistance, adhesiveness to a substrate and alkaline resistance, and to provide a surface treatment agent, an article and an optical article.A-Rf1-B (1).