Provided is an oligonucleic acid analog which contains, as at least one structural unit thereof, a modified nucleic acid monomer compound which is a ring-open nucleoside having a cleaved carbon-carbon bond between the 2′ and 3′ positions and a substituent hydroxymethyl group at the 4′ position. When used as siRNA, the oligonucleic acid analog exhibits superior biological stability and target gene expression inhibiting activity. The oligonucleic acid analog can be used in antisense methods, ribozyme methods, and decoy methods, etc., can be used as a nucleic acid aptamer, and can also be used as a nucleic acid probe or molecular beacon, etc., or in genetic diagnostics, etc.