The waterproof grease composition of the present application comprises a base oil that comprises at least one of mineral oil and synthetic oil and having a kinematic viscosity of from 10 to 400 mm2/sec at 40° C., a thickener, and a waterproofness-imparting agent selected from surfactant, metal soap, fluorine-containing water repellent, silicone-type water repellent, pH-controlling agent, wax, polymer and graphite. Accordingly, it may prevent peeling such as white structure peeling even when water is mixed therein.The wheel-supporting roller bearing of the invention comprises an inner member having a raceway surface in the outer peripheral surface thereof, an outer member having, in the inner peripheral surface thereof, a raceway surface that faces the raceway surface of the inner member and disposed outside the inner member, and plural rolling elements rollably disposed between the two raceway surfaces, and which contains the above waterproof grease composition sealed therein; and this has excellent waterproofness.