Sealing structures (7, 8) are installed to seal opposite open ends of an annular bearing space between an outer member (1) and an inner member (2). A wheel mounting flange (2a) of the inner member (2) is provided with a shielding plate (11). The shielding plate (11) has a tubular wall (11a) forming a labyrinth seal (10) in the vicinity of an outer peripheral surface (1b) of an outboard end of the outer member (1). The outer peripheral surface (1b) is provided with a guide structure (1c) for guiding downwardly water, flowing in between the shielding plate (11) and the outer member (1) along the flange (2a) and the shielding plate (11), and water flowing in between the shielding plate (11) and the outer member (1) along the outer peripheral surface of the outer member (1). The guide structure (1c) is constituted by an annular groove or projection.