A conductivity sensor for measuring conductivity of a medium surrounding the conductivity sensor, including: a first toroidal coil, which surrounds a first traversing opening for accommodating the medium and which serves for inducing an electrical current in the medium, a second toroidal coil, which surrounds a second traversing opening for accommodating the medium and which serves for registering a magnetic field produced by the induced electrical current. At least one of the toroidal coils has a plurality of first conductor sections, which extend in a plane of a multi-ply circuit card, a plurality of second conductor sections, which extend in a second plane of the circuit card, and a plurality of first vias, which connect the first conductor sections with the second conductor sections. The first conductor sections, the second conductor sections and the vias together form the windings of a toroidal coil, wherein at least one toroidal coil is surrounded by a plurality of internally metallized bores, which are arranged in such a manner, that they act as electrical shielding of the toroidal coil.