A silicon-containing copolymer which comprises units represented by the general formulae (I) and (II) as essential units and ones represented by the general formulae (III) to (VII) as optional units and has a number average molecular weight of 500 to 1,000,000, —(—R1R2Si—A —)p—  (I) —(—R3R4Si—R7—SiR5R6—)q—  (II) —(—R8Si—A—)r—  (III) —(—R9(H)Si—A—)s—  (IV) —(—R1R2Si—NH—R10—NH—)t—  (V) —(—R8Si—NH—R10—NH—)u—  (VI) —(—R9(H)Si—NH—R10—NH—)w—  (VII) wherein A is NH or oxygen, the proportions of these satisfying the relationship 0.01≦Si—O/(Si—N+Si—O)≦0.99.