To provide a grinding machine capable of automatically detaching and re-attaching a grinding tool.

The grinding machine is provided with: a work holding member for holding a work; a wheel spindle head 3 movably disposed at the work holding member; a grinding wheel spindle 3a rotatably journaled to the wheel spindle head 3; an extension spindle 8 including an extension wheel spindle 8a detachably connected by providing a connection part R to the grinding wheel spindle 3a for a grinding tool machining a work to be fitted in the grinding machine in which a tool holder of the grinding tool is manually detached and reattached from/to the grinding wheel spindle 3a and an extension outer cylinder 8e for rotatably supporting the extension wheel spindle 8a and clamped on the wheel spindle head 3; a clamp device C for clamping the extension outer cylinder 8e to the wheel spindle head 3; and a centering part B attached to an outer cylinder head 3d of the wheel spindle head 3 and the extension outer cylinder 8e and for performing centering between the grinding wheel spindle 3a and the extension wheel spindle 8a.