A retaining seal material includes inorganic fibers, inorganic particles adhering to surfaces of the inorganic fibers, a first principal surface, and a second principal surface. A mean particle diameter of the inorganic particles in a vicinity of a center in a thickness direction of the retaining seal material is smaller than at least one of a mean particle diameter of the inorganic particles in a vicinity of the first principal surface and a mean particle diameter of the inorganic particles in a vicinity of the second principal surface.