PURPOSE: To lubricate a ball bearing which rotates at a high rotation certainly, and obviate necessity of a seal member provided between a lube oil supplying system and a rotary shaft so as to eliminate loss caused by sliding resistance.

CONSTITUTION: A lube oil passage 9 is formed by an inner cylinder 4 constituting a rotary shaft 1, and a bearing oil supplying hole 14 communicated with the oil reservoir part 10 on the top end of the lube oil passage 9 is opened on an or bit surface after penetrating the inner wheel 2a of a ball bearing 2. A lube oil nozzle 26 for injecting lube oil is inserted in the opening end of the lube oil passage 9 serving as a nozzle inserting part 11 in no-contact condition. A spiral streak 30 is formed on the inner circumferential surface of the lube oil passage 9 so as to feed lube oil to the oil reservoir part 10 side in association with rotation of the rotary shaft 1.