Abstract: Device for checking the vertical alignment of the upper and lower internal equipment of a pressurized water nuclear reactor comprising a vertical tubular body (14) onto which are fixed a flange (15) for centering the upper internal equipment on the upper plate (2), a flange (16) for centering on the upper core plate (3) and a lower end member (17) which enables the device (1) to be centered on the lower core plate (4). A metal wire (24) is aligned along the axis of the tubular body (14), its lower part being fixed to the end member (17) and its upper part to a fitting (20) which can move in two horizontal directions at 90.degree.. Electrodes are arranged in the flange (16) around the wire (24), to detect, by virtue of an electrical monitoring circuit (90), contact between the wire (24) and the electrodes when the upper part of the wire is moved by the fitting (20).