A laser processing unit includes a torch that is automatically mounted on a processing head. The torch has an assist gas chamber and a balance chamber, and the assist gas is supplied to the chambers. Constantly, equal pressures act on the both chambers, and an upward first force and a downward second force that act on a lens holder offset each other because pressure reception areas are equal. Gas is supplied to a gas spring chamber to provide a constant upward force to the lens holder. A driving device has an arm driven by a servomotor. The arm is in contact with a pin and the lens holder is moved to a predetermined position for automatically adjusting a focus position FC of a collective lens.