A perfluoropolyether-polyorganosiloxane block copolymer represented by the following formula (1), the block copolymer having a number average molecular weight, reduced to polystyrene, of from 2,000 to 20,000, W2-Q-Rf-Q-(W1-Q-Rf-Q)g-W2  (1) wherein Rf is a perfluoropolyether block, W1 is a polyorganosiloxane block having at least one group represented by the following formula (2) wherein X is a hydrolyzable group, R1 is a C1-4 alkyl group or a phenyl group, y is an integer of from 1 to 5, and a is 2 or 3, Q is a C2-12 divalent connecting group which may contain an oxygen atom and/or nitrogen atom, W2 is an organosiloxane residue having a group represented by the aforesaid formula (1), and g is an integer of from 1 to 11.