Composition for forming resist underlayer film, resist underlayer film and resist underlayer film-forming method, and pattern-forming method

A composition for forming a resist underlayer film, comprising a polymer having a structural unit represented by a formula (1):

• wherein in the formula (1),
• Ar1, Ar2, Ar3 and Ar4 each independently represent a divalent aromatic hydrocarbon group or a divalent heteroaromatic group, wherein a part or all of hydrogen atoms included in the divalent aromatic hydrocarbon group and the divalent heteroaromatic group represented by Ar1, Ar2, Ar3 or Ar4 are unsubstituted or substituted;
• R1 represents a single bond or a divalent hydrocarbon group having 1 to 20 carbon atoms, wherein a part or all of hydrogen atoms included in the divalent hydrocarbon group represented by R1 are unsubstituted or substituted, and wherein the divalent hydrocarbon group represented by R1 has or does not have an ester group, an ether group or a carbonyl group in a structure thereof;
• Y represents a carbonyl group or a sulfonyl group;
• m is 0 or 1;
• and
• n is 0 or 1.