A method produces a hairpin single-stranded RNA molecule capable of inhibiting expression of a target gene, the method including: (i) an annealing step of annealing a first single-stranded oligoRNA molecule and a second single-stranded oligoRNA molecule; and (ii) a ligation step of ligating 3′ end of the first single-stranded oligoRNA molecule and 5′ end of the second single-stranded oligoRNA molecule by an Rnl2 family ligase, wherein a sequence produced by ligating the first single-stranded oligoRNA molecule and the second single-stranded oligoRNA molecule includes a gene expression-inhibiting sequence for the target gene.