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: BX represents a nucleobase optionally having a protecting group; and R1 is a substituent represented by general formula (2), wherein: R11, R12 and R13 are the same or different and each represents hydrogen or alkoxy; R2a and R2b are the same or different and each represents alkyl, or R2a and R2b 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 WG1 and WG2 are the same or different and each represents an electron-withdrawing group.