The invention relates to a passive pressure sensor (501) for being introduced into the circulatory system of a human being and for being wirelessly read out by an outside reading system. The pressure sensor comprises a casing (502) with a diffusion blocking layer for maintaining a predetermined pressure within the casing and a magneto-mechanical oscillator with a magnetic object (508) providing a permanent magnetic moment. The magneto-mechanical oscillator transduces an external magnetic or electromagnetic excitation field into a mechanical oscillation of the magnetic object, wherein at least a part of the casing is flexible for allowing to transduce external pressure changes into changes of the mechanical oscillation of the magnetic object. The pressure sensor can be very small and nevertheless provide high quality pressure sensing.