Abstract: An apparatus for immobilizing a patient so as to perform on this patient medical and paramedical procedures, in particular x-rays, this apparatus having a holding device that is upwardly open and set up to receive the patient (8) when laying down.It has a rigid, preferably rectangular, oblong base (1) of a material permeable to x-rays, and has sides (3 and 4) that are closed and foldable against the patient, the sides being of a material permeable to x-rays and having the general shape of channels attached to the long edges of the base and of a convexity directed when they are against the patient toward the longitudinal axis of the base, tightening elements (5) permeable to x-rays being also provided to solidly maintain these channels in lateral contact with the patent.