PURPOSE: To process a work piece precisely at the same time by measuring the displacement of the interval between two main spindles by the thermal displacement of the spindle head, and parallel shifting two fine moving tables where the workpiece is mounted, according to the measurement results, and compensating them following the thermal displacement.

CONSTITUTION: The two main spindles 7 and 8 installed at the spindle head 6 are rotated at the same time, and a workpiece in of the same shape in the right and left installed on the right and left fine moving tables 16 and 17 supported by a table 3 is processed at the same time. When the spindle head 6 expands by heat during the processing, and the interval 9 between the main spindles 7 and 8 extends, the quantity of extension is measured based on the center line 7a of the main spindle 7 with the displecement gage 12 and the detection block 13 provided at the spindle head 6, and is input into a controller. The motor 18 of the fine moving table 16 is driven with that command, and it is shifted left by half the quantity of elongation, also the table 17 also is shifted right similarly, and the quantity of its shifting is input into a controller based on the table 16 with the displacement gage 25. Accordingly, the fine moving tables 16 and 17 also are displaced according to the displacement of the main spindles 7 and 8, and the workpiece is processed at the same time, with the displacement compensated.