In a transmission antenna provided with an antenna coil outfitted with a core whereon a coil is mounted by winding, and with a capacitor connected to the aforementioned coil forming a serial resonance circuit between the aforementioned antenna coil inductance, the aforementioned antenna coil is outfitted with small core screw with a core smaller than the aforementioned core, and with a joining material that magnetically joins the aforementioned screw to the aforementioned core and with a screw hole that serves as a non-magnetic distance adjuster whereby the distance between the aforementioned core and the aforementioned screw is adjustable, and the resonance frequency is set by adjusting the thread volume of the aforementioned screw.