A method for preparing a high-pressure phase cubic spinel-type silicon nitride includes housing a molding containing low-pressure phase silicon nitride powder and a metal powder in a cylindrical container, arranging an explosive in the cylindrical container so as to surround the molding, and exploding the explosive to compress the molding. An X-ray diffraction pattern of the high-pressure phase cubic spinel-type silicon nitride produced according to the method of the present invention shows a maximum peak having a full width at half maximum of 0.65 degrees or less. TG-DTA analysis of the cubic spinel-type silicon nitride shows a weight change starting temperature of 700 to 1100.degree. C.