An object of the present invention is to provide a useful and novel phosphoramidite compound for the synthesis of oligo-RNA. A phosphoramidite compound represented by general formula (1), wherein: B.sub.X represents a nucleobase optionally having a protecting group; and R.sup.1 is a substituent represented by general formula (2), wherein: R.sup.11, R.sup.12 and R.sup.13 are the same or different and each represents hydrogen or alkoxy; R.sup.2a and R.sup.2b are the same or different and each represents alkyl, or R.sup.2a and R.sup.2b taken together with the adjacent nitrogen atom may form a 5- to 6-membered saturated amino cyclic group, the amino cyclic group optionally having an oxygen or sulfur atom as a ring-composing member in addition to the adjacent nitrogen atom; and WG.sup.1 and WG.sup.2 are the same or different and each represents an electron-withdrawing group.