An apparatus for demolishing a reactor shield wall which includes a pillar extending from the top side of the reactor toward the bottom of the reactor, upper support means for supporting a tip portion of the pillar at the top side of the reactor, and cutting means mounted to the pillar so as to be movable upwardly and downwardly along the pillar wherein said pillar is rotatably supported by said upper support means, and said upper support means is provided with drive means for rotating the pillar, said pillar extending generally along the axis of the reactor interior and having at the lower end thereof a lower support means, said lower support means having a body rotatably fitted on the lower end of the pillar, a plurality of arms extending horizontally radially from said body, said arms being retractably projectable, and drive means for projecting and retracting said arms, whereby the front end of each arm is pressed against the inner surface of the reactor shield wall, and wherein said pillar (2) comprises an upper pillar (2a) mounted to the upper support means (3), a lower pillar (2b) to which the lower support means (4') are mounted, and intermediate pillars (22, 23) removably fitted between said upper pillar (2a) and lower pillar (2b), the lower end portion of said upper pillar (2a) and the upper end portion of said lower pillar having connecting means (20, 21) for connecting both pillars.