The present invention provides a surface modifier for an inorganic solid, of which a small amount improves efficiently and significantly the affinity of the inorganic solid with a medium, thus obtaining a composition having no bad effects even if the inorganic solid is blended in a large amount. That is, the present invention provides a surface modifier for an inorganic solid, which comprises a polyether having a monomeric unit represented by the formula (I); a inorganic solid having at least a part of its surface coated with the surface modifier; and a composition comprising the surface modifier and a inorganic solid: ##STR00001## wherein R.sup.1 represents a hydrogen atom, a C.sub.1-50 hydrocarbon or fluoroalkyl group, a C.sub.6-50 fluoroaryl group, a Si.sub.1-50 siloxysilylpropyl group, etc.