1. A plasma torch with a structure capable of performing reversed polarity/straight polarity operation, wherein the plasma torch is coupled to a melter and melts a waste material such as radioactive waste or industrial waste by generating and sustaining a plasma arc between electrodes, the plasma torch comprising:a rear electrode provided inside a torch pipe and electrically connected to become one of an anode or a cathode;
a front electrode provided at a front end of the torch pipe at a position adjacent to a front end of the rear electrode and electrically connected to become another one of the anode or the cathode; and
a first-shaft torch feed means configured to linearly feed the plasma torch forward and backward in a direction parallel to a lengthwise direction of the plasma torch,
wherein electrical connections of the rear and front electrodes are switchable with each other so that the plasma torch operates as a reversed polarity plasma torch or a straight polarity plasma torch,
wherein the first-shaft torch feed means includes:
a first-shaft linear motion (LM) guide guiding the plasma torch to move linearly; and
a first-shaft guide block provided on the first-shaft LM guide to be movable linearly and fixedly supporting on an upper portion thereof the plasma torch; and
wherein a surface of the first-shaft guide block contacts a surface of the plasma torch.