In an extrusion or injection molding composition comprising water-insoluble particles, a water-soluble binder, and water, true spherical particles having an average particle size of 0.2-20 .mu.m are used as the water-insoluble particles. The addition of a small amount of a binder to water-insoluble particles as the substrate facilitates molding operation and enables to produce a molded part featuring a shape stability after molding.