The present invention relates to a circular single-stranded nucleic acid complex comprising a sense strand RNA sequence, an antisense strand RNA sequence complementary to the sense strand RNA sequence, and two identical or different loop moieties connecting the sense and antisense strands, wherein the sense and antisense strands form a stem by pairing, and the loop moieties are selected from the group consisting of polyalkylene glycol, DNA, DNA-RNA chimera, a derivative thereof, and a combination thereof.