PURPOSE: To prevent thermal deformation of a main spindle head caused by a temperature rise in lube oil, and improve the working accuracy by preventing the lube oil from staying on the under surface of a gear chamber of the main spindle head after bubricating a bearing.

CONSTITUTION: Lube oil discharge passages 39 and 40 are opened in a gear chamber 31 formed in a main shaft head 15, and an oil pan 46 is installed under the main shaft head 15 so as to correspond to the under part of this gear chamber 31, and discharge holes 42 and 43 to communicate the gear chamber 31 and the oil pan 46 with each other, are formed under the main shaft head 15.