A monoconical antenna comprises: a substantially conical concavity formed in one end face of a dielectric; a radiation electrode provided on the surface of the concavity; and a ground conductor provided in proximity to and substantially in parallel with the other end face opposite the one end face of the dielectric. The monoconical antenna is so constituted that electrical signals are fed to between the near vertex region of the radiation electrode and the region of the ground conductor. The half-cone angle α of the substantially conical concavity formed in the one end face of the dielectric is determined by a predetermined rule corresponding to relative dielectric constant ∈r. Thus, the quality of wideband characteristics inherent in the monoconical antenna can be sufficiently maintained, and further size reduction can be accomplished by dielectric loading.