An antenna structure includes a radiation conductor made of a metal plate that is supported by legs provided upright on a surface of a dielectric substrate and is spaced apart from the surface. The radiation conductor is circularly shaped such that the contour of the radiation conductor conforms to the outer periphery of the dielectric substrate and such that the radiation conductor includes an open portion. One of the legs functions as a feeding terminal and the other functions as a grounding terminal, whereby the radiation conductor operates as a dipole antenna. The radiation conductor has slots for adjusting impedance. Impedance can be adjusted by changing the length of the slots.