The invention relates to a case for screening magnetic fields, comprising at least two parts, in particular, a lower part (4, 5) and a bell-shaped upper part (3). These have such a form as to be suitable to enclose a timepiece (1). The upper piece (3) is mounted such that when mounted on the timepiece it is displaceable in a plane parallel to the timepiece (1), whilst it is essentially impossible to displace the same in the direction perpendicular to said plane.