PURPOSE: To provide a compound which comprises a polyribonucleotide having a specific base sequence, has a thermodynamically stable loop structure, exhibits ribozyme activity and has preventive and therapeutic effects against diseases originating from RNA such as AIDS.

CONSTITUTION: Nucleoside-3'-phosphoroamidites having 5'-hydroxyl protected with a dimethoxytolyl group and 2'-hydroxyl protected with t-butylmethylsilyl group are linked to the support in order by the solid phase method using an autosynthesizer and CPG(Controlled Pore Glass) as a support, them the CPG to which the synthesized oligonucleotide is bonded is treated with aqueous ammonia to eliminate the oligonucleotide from the support. The product is purified by chromatography to give the objective polyribonucleotide which has a thermodynamically stable loop structure shown by the formula (S, V are arbitrary bases; Q1 to Qn are C, G; R1 to Rn are the bases complementary to Q1 to Qn; W1 to Wk, X1 to Xm are arbitrary bases; k, m, l are 1 to 10), and has the ribozyme activity.