Double-stranded modified siRNA targeting a RecQL1 helicase gene includes a sense strand including the nucleotide sequence shown in SEQ ID NO: 1, and an antisense strand including the nucleotide sequence shown in SEQ ID NO: 2, wherein the sense strand includes 2′-substituted nucleotides at positions 2, 3, 4 and 13 in the nucleotide sequence shown in SEQ ID NO: 1, the sense strand further includes a 2′-substituted nucleotide(s) at one or more positions selected from the group consisting of positions 12, 14, 17, 18 and 19 in the nucleotide sequence shown in SEQ ID NO: 1, wherein the position 2′ of the 2′-substituted nucleotides is —R1, —OR1, —R2OR1, —OR2OR1 or —R3OR2OR1, wherein R1 represents a C1-4 alkyl group, and R2 and R3 independently represent a C1-3 alkylene group.