A self-acting air bearing wherein an abrasion-generated substance is generated by rotation of a shaft and a bearing and is used as a lubricant. As a result, a motor of low cost and stable rotation in which it is not necessary to insert special lubricant into the shaft or the bearing previously can be achieved. Moreover, in a polygon motor having such a lubrication structure, since polluting of a writing system is not feared, reliability for a long time period can be obtained.