To provide an intermediate having a good antisense or anti-gene activity, allowing to produce nonnatural type oligonucleotide analogs stable in vivo and also an intermediate to yield 2-5A analogs.

This nucleic acid reagent comprises, a compound represented by formula 1 [wherein, R1 and R3 are the same or different and are each hydrogen or protective group and the like for the hydroxy group in the synthesis of a nucleic acid; R2 is hydrogen, or a protective group for the 1-6C alkyl group or the amino group in the synthesis of a nucleic acid, and B is purin-9-yl or 2-oxo-1,2-dihydoropyrimidin-1-yl group which may have the substituent selected from the α groups (α groups are a halogen, a 1-6C alkyl, hydroxy, mercapto, amino group or the like)], or its pharmacologically acceptable salt.