An antenna device for a portable terminal, comprising: a circuit board having a conductive layer formed on a surface; a first slit formed by partially removing the conductive layer in a position adjacent to one side of the circuit board, the first slit extending in parallel with a lateral periphery of the circuit board; a radiation portion (113b, 113c, 113d) comprising a part of the conductive layer positioned on the lateral periphery of the circuit board in one side of the first slit, the first slit having one end open to a periphery of the conductive layer, and another end positioned within the conductive layer and closed; and a feed line placed on the first slit and adapted to feed the radiation portion (113b, 113c, 113d) from the other side of the first slit, wherein the radiation portion (113b, 113c, 113d) further comprises a second slit extending from the first slit to the lateral periphery of the circuit board across part of the radiation portion (113b, 113c, 113d); and a frequency adjustment element placed on the second slit and adapted to connect in series the parts of the radiation portion (113b, 113c, 113d) divided by the second slit and positioned on respective sides of the second slit; and wherein the radiation portion (113b, 113c, 113d) comprises a first radiation portion on the one side of the first slit and connecting to the conductive layer at the other side of the first slit, and a second radiation portion separated from the first radiation portion by the second slit and from the rest of the conductive layer by the first slit, the feed line being connected to the first radiation portion; characterised in that the antenna device further comprises a second radiation pattern formed on a different surface of the circuit board; and a second via-hole formed through the circuit board, the second radiation pattern being connected to the second radiation portion through the second via-hole and formed to have a shape that extends so as to surround partially at least one side, the other end and the other side of the first slit; and in that the antenna device is adapted to generate a current flow around the first slit in a direction following a signal power flowing through the second radiation pattern when a transmission signal is applied to the feed line. a circuit board having a conductive layer formed on a surface; a first slit formed by partially removing the conductive layer in a position adjacent to one side of the circuit board, the first slit extending in parallel with a lateral periphery of the circuit board; a radiation portion (113b, 113c, 113d) comprising a part of the conductive layer positioned on the lateral periphery of the circuit board in one side of the first slit, the first slit having one end open to a periphery of the conductive layer, and another end positioned within the conductive layer and closed; and a feed line placed on the first slit and adapted to feed the radiation portion (113b, 113c, 113d) from the other side of the first slit, wherein the radiation portion (113b, 113c, 113d) further comprises a second slit extending from the first slit to the lateral periphery of the circuit board across part of the radiation portion (113b, 113c, 113d); and a frequency adjustment element placed on the second slit and adapted to connect in series the parts of the radiation portion (113b, 113c, 113d) divided by the second slit and positioned on respective sides of the second slit; and wherein the radiation portion (113b, 113c, 113d) comprises a first radiation portion on the one side of the first slit and connecting to the conductive layer at the other side of the first slit, and a second radiation portion separated from the first radiation portion by the second slit and from the rest of the conductive layer by the first slit, the feed line being connected to the first radiation portion; characterised in that the antenna device further comprises a second radiation pattern formed on a different surface of the circuit board; and a second via-hole formed through the circuit board, the second radiation pattern being connected to the second radiation portion through the second via-hole and formed to have a shape that extends so as to surround partially at least one side, the other end and the other side of the first slit; and in that the antenna device is adapted to generate a current flow around the first slit in a direction following a signal power flowing through the second radiation pattern when a transmission signal is applied to the feed line.