An effective contact length in race surfaces formed on grooved races cut in an outer ring and an inner ring is made as greater as permitted to improve the load-carrying capacity, and correspondingly guide parts born against axially opposite ends of a roller are made as less as possible in width to reduce frictional contact resistance applied to the axially opposite ends of the roller, thereby preventing a skew of the roller. An inside circular surface on the outer ring is staggered or different in level on opposite sides of a outside grooved race to make greater the race surface than an effective contact length of circular rolling surfaces of the rollers and correspondingly less the guide part in width than the race surface.