In a PET (Positron Emission Tomography)-MRI (Magnetic Resonance Imaging) apparatus of an embodiment, a transmitting radio frequency coil applies a radio frequency magnetic field on a subject placed in a static magnetic field. A detector is formed in a ring shape, disposed on a side adjacent to an outer circumference of the transmitting radio frequency coil, includes at least two PET detectors disposed with a space therebetween in an axial direction of a bore so as to interpose the magnetic field center of the static magnetic field therebetween, and detects gamma rays emitted from positron emitting radionuclides injected into the subject. Radio frequency shields are each formed in an approximately cylindrical shape, disposed between the transmitting radio frequency coil and the detector, and shield the radio frequency magnetic field generated by the transmitting radio frequency coil.