A positron emission tomography (PET)-magnetic resonance imaging (MRI) apparatus according to an embodiment includes a gantry having a static magnetic field magnet, a gradient coil, and a radio frequency coil, a PET detector, and a moving mechanism. The static magnetic field magnet generates a static magnetic field in a bore having an approximately cylindrical shape. The gradient coil is disposed on an inner circumference side of the static magnetic field magnet and applies a gradient magnetic field to an object disposed in the bore. The radio frequency coil is disposed on an inner circumference side of the gradient coil and applies a radio frequency magnetic field to the object. The PET detector detects gamma rays emitted from a positron-emitting radionuclide injected into the object. The moving mechanism causes the PET detector in the gantry along the axial direction of the bore.