To provide a boring device which is simple in structure and improved in machining accuracy.

The boring device 40 functions to machine an inner peripheral surface of a hole formed in a workpiece 30 by relatively moving a table 2 which is arranged on a bed 1 and has the workpiece 30 mounted thereon, and a main shaft head 4 which holds a tool and is rotatable, with respect to each other. Herein the boring device comprises an outrigger device 5 which is a fixing means for supporting and fixing the main shaft head 4 to the workpiece 30, and therefore, the main shaft head 4 is prevented from deflecting, thereby improving the machining accuracy of the workpiece 30.