Reactor shielding wall dismantling apparatus provided with a support (2) disposed in such a manner as to extend from the upper portion of a reactor to its bottom, an upper support means (3) for supporting the upper part of this support (2) at the upper portion of the reactor, and cutting means (5, 6) disposed vertically movably on the support (2). The support (2) is rotatably supported by the support means (3). The support means (3) is further provided with a rotating means (30, 31, 32) for rotating the support (2). Thus, when the support (2) is rotated, the cutting means (5, 6) can be rotated inside the reactor. Since the rotating means is disposed on the upper portion of the reactor, its maintenance and inspection is easy.