An ion implanter includes: a plurality of devices which are disposed along a beamline along which an ion beam is transported; a plurality of neutron ray measuring instruments which are disposed at a plurality of positions in the vicinity of the beamline and measure a neutron ray from a neutron ray source which is generated in the beamline due to collision of a high-energy ion beam; and a control device which monitors at least one of the plurality of devices, based on a plurality of measurement values measured by the plurality of neutron ray measuring instruments.