A coil device for use in a transcranial magnetic stimulation device that has a coil provided so as to face the surface of the head of a human, and stimulates neurons by producing an electric current from an induced electric field in a magnetic stimulation target region inside the brain as a result of electromagnetic induction, the coil device being equipped with a coil configured by winding a conductive wire along a prescribed reference surface, and a magnetic body that: is positioned so as to face the coil and be opposite the head from the coil with the coil sandwiched therebetween； has an electric current flowing therethrough from an induced electric field when the coil is driven； and increases the electric current from the induced electric field that flows through the magnetic stimulation target region of the brain as a result of the electric current produced by the induced electric field, in comparison to when no magnetic body is present.
