To open up new possibilities for generating sufficient lubrication without keeping a lubrication mechanism large toward a mechanism arranged inside an actuator 2 and for easily discharging foreign bodies and starting drive.

A second vent 8d has a discharge capacity which is lower than a supply capacity of a supply port 8b. A first vent 8c can sufficiently discharge an oil feed volume Vo from the supply port 8b so that an oil level reaches an up-and-down width region W inside which is predetermined to become stable at the time of feeding lubrication oil. Consequently, even one supply port 8b can feed a sufficient lubrication without upsizing the lubrication mechanism. Further, when the lube oil supply is suspended, almost all the lube oils are emitted from the second vent 8d to the outside due to oils'own weight. Accordingly, there is no need to inflow the lube oil at a speed of flow to wash out foreign materials from plural supply ports to every hole and corner, and foreign matters are discharged without upsizing the lubrication mechanism. As almost no lube oil exists at the time of the actuator 2 starting drive and excessive friction is not caused even in cold temperature, starting drive is easily done.