It is desired to prevent generation of negative pressure in the bearing space and to prevent entry of foreign matter such as water into the bearing. A bearing is provided which includes an inner race (1), an outer race (3), rolling elements (5) disposed between raceways (2 and 4) of the inner and outer races and retained by a retainer (6), and seal assemblies (7) provided at the respective ends of the outer race (3). Each seal assembly (7) includes a seal case (9) fitted on one end of the outer race (3), and an oil seal (11) mounted in the seal case. A check valve (20) is mounted in the seal case (9) which allows only a flow of gas from outside the bearing into the bearing space. A filter (29) is mounted in a flow passage in the check valve which allows passage of gas and prevents passage of liquid and solids.