A bit head retaining system includes a bit head having an upper end and a working face, and a retention sleeve adapted to be connected to an end of a drill casing and having an internal opening in which at least a portion of the bit head above the working face is disposed, the bit head being secured to the retention sleeve so as to be axially immovable relative to the retention sleeve. A sensor is disposed in an image formation region and detects a pattern on the transfer belt. A first well may be disposed on the substrate and doped with a second dopant. The amount of information of a digital video signal inputted to a source signal line drive circuit is reduced. The ladle supplies molten metal, from which slag has been removed in the crucible, into molds.