Abstract: In apparatus for exchanging a control rod drive mechanism of a nuclear reactor of the type comprising a horizontal platform supported to be rotatable in a working chamber disposed below a reactor pressure vessel and a traveling carriage traveling on a rail assembly laid on the platform, there is provided a beam attached to the traveling carriage to be swingable between the vertical and horizontal positions and provided with a carrier for vertically moving the control rod drive mechanism. A holding device is secured to the beam so as to hold the control rod drive mechanism when the beam is moved to the horizontal position. The bolts which are used to connect the control rod drive mechanism to a housing in the pressure vessel are loosened and clamped by a bolt mounting device, which is conveyed in and out of the passage of movement of the carrier.