Means are provided for effectively restricting or preventing occurrence of abnormal noise under cold ambient. The means are durable, cost effective and also keep the function of a pulley. A ball bearing 2 of a single row type includes an outer ring 2a and balls 2c. The outer ring 2a and each ball 2c are contacted with each other at two points. An oil groove 5 is formed at a position adjacent to the contact positions P and Q on the raceway surface between the raceway surface 3a of the outer ring and the balls 2c. The ball bearing 2 is fitted within or made integral with the inner diameter of a pulley body 1 having a peripheral surface 1e to be contacted with a belt.