A tapered roller bearing is used in a planetary gear speed reducer, and includes an outer race, and a retainer formed with a first annular portion, a second annular portion, and pillars. At least one retainer guiding surface (raceway) which guides the retainer in a radial direction is formed only on an inner periphery of the outer race. Guided surfaces are formed at least either on the pillars or on the first and second annular portions, and guided by the at least one retainer guiding surface. Therefore, it is possible to guide an axial central portion or both side portions of the retainer, to make the retainer less likely to tilt due to a centrifugal force, and to apply a preload to both side surfaces of the outer race.