The main spindle unit has a main spindle, a front bearing, a rear bearing, a front case for covering the front bearing, and a rear case for covering the rear bearing. The front case has a front body part for holding the front bearing and a front flange part extending radially outward from the front body part. The rear case has a rear body part for holding the rear bearing and a rear flange part extending radially outward from the rear body part. A front mounted surface, which can be brought into contact with a first mounting surface of a unit support table, is formed on the front flange part so as to face the front side thereof. A rear mounted surface, which contacts a second mounting surface of the unit support table, is formed on the rear flange part so as to face the front side thereof.