To enable smooth operations of a jack even during the excavation of a hard layer, etc., and enhance the durability of the jack by preventing forces in directions other than the axial direction of a disc cutter from acting on the jack for displacing the disc cutter.

A hydraulic jack 15 for adjusting a protrusive margin of a disc holder 13 is composed of a cylinder 16 and a piston-piston rod assembly 17; the disc holder 13 is held so as to be prevented from being turned with respect to a guide cylinder 14 by a key groove 22 and a key 23; a minute gap is formed in an engaging part in which the key 23 is engaged with the key groove 22; an inward locking protrusive streak 26 is continuously provided at the tip of a piston rod part 17b, held in a state of being sandwiched between an end plate 13c and a stopper ring 24, and engaged in an approximately loosely-fitted manner while having the minute gap; and the disc holder 13 is coupled to the assembly 17 in a state of being bendable and twistable by the minute gap.