A protein having an activity of degrading a dsRNA, namely, being capable of acting on a long-chain dsRNA to form a dsRNA of a definite length; a method of efficiently preparing a dsRNA of a definite length which comprises treating a dsRNA with the protein having an activity of degrading a dsRNA in the coexistence of a protein having an activity of binding to a nucleic acid such as a protein having an RNA-binding activity; and a method of using the protein having an activity of binding to a nucleic acid to elevate the efficiency in an RNA synthesis reaction typified by dsRNA synthesis.