To supply a sufficient quantity of a lubricating oil to a bearing by a small-sized substituting means when a motor-driven pump is stopped due to abnormality.

When a machine tool is operated normally, the motor-driven pump 7 supplies the lubricating oil from a tank 1 through an oil feeding pipe 3 to a main spindle unit 2. When the motor-driven pump 7 is stopped due to abnormal condition such as power failure or the like, an electromagnetic switch device 23 is opened to demagnetize a solenoid valve 8, so that an air driving pump 11 is connected to an air source 9. Thus, the air driving pump 11 is immediately started instead of the motor-driven pump 7 to supply the lubricating oil from the tank 1 through a substituting pipe 13 and the oil feeding pipe 3 to the main spindle unit 2, thereby lubricating a main spindle bearing until the main spindle is stopped.