Antenna structure having stable properties and headset

An antenna structure for a speaker-containing unit to be fitted to an ear, the antenna structure comprising:

• a circuit substrate having a first principal surface and a second principal surface;
• a radiation conductor made of a flat metal plate and disposed at a position spaced apart from the first principal surface of the circuit substrate, the radiation conductor being provided along a periphery of the circuit substrate with an open portion along the periphery, the radiation conductor having an outer shape substantially same as an outer shape the circuit substrate;
• a first leg and a second leg for supporting the radiation conductor above the first principal surface, the first leg serving as a feeding terminal, the second leg serving as a grounding terminal;
• and
• a grounding conductor provided on one of or each of the first and the second principal surfaces in a region facing the radiation conductor, wherein the radiation conductor serves as a dipole antenna,
• wherein the radiation conductor has at least one slot for adjusting impedance, and
• wherein said at least one slot comprises a first slot and a second slot having predetermined dimensions, the first slot being defined by a first extending portion that extends from an inner periphery of the radiation conductor and connects to the first leg and by a first slot-defining piece that extends from the first extending portion in an opposite direction to the open portion of the radiation conductor, the second slot being defined by a second extending portion that extends from the inner periphery of the radiation conductor and connects to the second leg and by a second slot-defining piece that extends from the second extending portion in the opposite direction to the open portion.