A polypeptide having an RNase III activity with which the length of a dsRNA degradation product can be easily controlled depending on reaction conditions and, in preparing a dsRNA having a length allowing it to serve as an siRNA in RNA interference, a low-molecular weight product having little RNA interferring effect is scarcely formed; a method of degrading a dsRNA with the use of the above polypeptide; and a composition and a kit for the above method.