A producing method of a porous Si.sub.3N.sub.4 having high porosity and formed of Si.sub.3N.sub.4 particles having a high aspect ratio includes the following steps. A compound of a rare earth element as a first sintering agent is mixed in an amount of 7.5 45 parts by mass, in terms of an oxide of the element, with respect to 100 parts by mass of Si powder to obtain mixed powder. A binder is added to the mixed powder, which is then molded into a molded body. The molded body is heated in a nitrogen atmosphere to 300 500.degree. C. to remove the binder. The binder-removed body is heated in a nitrogen atmosphere to 1350 1500.degree. C. for nitriding. The nitrided body is then sintered at 1750 1900.degree. C. at a nitrogen pressure of 0.1 1 atmosphere.