A surgical apparatus having sufficient rigidity in a fixed mode and being freely operable in a steering mode includes one or more arm modules having at least one degree of freedom, each arm module including at least one steering cable to steer the arm module and a plurality of link units to form the at least one arm module, each link unit including a head disposed at an upper portion thereof and having a hemispherical shape, a seat formed at a lower portion thereof, and a sliding ball arranged at the seat, wherein the link units are arranged such that the head of one of the link units is seated on the seat of the other of the link units, and the head of one of the link units and the seat of the other of the link units are slid with respect to each other by the sliding ball.