An antenna device for a portable terminal, the antenna device comprising: a circuit board comprising a conductive layer on a first surface and a slit extending in a first direction on the first surface where the conductive layer is not present; an auxiliary board positioned over the slit and above the first surface of the circuit board; a conductor pattern formed on the auxiliary board; a feeding line connecting the conductive layer on a first and a second side of the slit across the slit, the first and second sides being defined on the first surface with respect to the first direction of the slit; and a connection terminal installed on the conductive layer on the second side of the slit; wherein a projection of the conductor pattern on the first surface of the circuit board at least partially encloses the slit in the plane of the first surface; wherein the conductive layer partially surrounds the slit; wherein a first end of the slit opens to a side edge of the conductive layer; wherein the conductor pattern comprises: a first extension portion positioned over the conductive layer on the first side of the slit extending parallel to the slit and electrically connected to the connection terminal over a surface of the auxiliary board opposite to a surface of the auxiliary board on which the conductor pattern is formed; a second extension portion extending from an end of the first extension portion, wherein a projection of the second extension on the first surface of the circuit board encloses a second end of the slit in the plane of the first surface; and a third extension portion at least a portion of which is positioned over the conductive layer on the second side of the slit extending from an end of the second extension portion in parallel with the slit; and wherein the feeding line is arranged to receive a transmission signal on the first side of the slit such that the transmission signal is delivered to the first extension portion through a path via the conductive layer and the connection terminal on the second side of the slit. a circuit board comprising a conductive layer on a first surface and a slit extending in a first direction on the first surface where the conductive layer is not present; an auxiliary board positioned over the slit and above the first surface of the circuit board; a conductor pattern formed on the auxiliary board; a feeding line connecting the conductive layer on a first and a second side of the slit across the slit, the first and second sides being defined on the first surface with respect to the first direction of the slit; and a connection terminal installed on the conductive layer on the second side of the slit; wherein a projection of the conductor pattern on the first surface of the circuit board at least partially encloses the slit in the plane of the first surface; wherein the conductive layer partially surrounds the slit; wherein a first end of the slit opens to a side edge of the conductive layer; wherein the conductor pattern comprises: a first extension portion positioned over the conductive layer on the first side of the slit extending parallel to the slit and electrically connected to the connection terminal over a surface of the auxiliary board opposite to a surface of the auxiliary board on which the conductor pattern is formed; a second extension portion extending from an end of the first extension portion, wherein a projection of the second extension on the first surface of the circuit board encloses a second end of the slit in the plane of the first surface; and a third extension portion at least a portion of which is positioned over the conductive layer on the second side of the slit extending from an end of the second extension portion in parallel with the slit; and a first extension portion positioned over the conductive layer on the first side of the slit extending parallel to the slit and electrically connected to the connection terminal over a surface of the auxiliary board opposite to a surface of the auxiliary board on which the conductor pattern is formed; a second extension portion extending from an end of the first extension portion, wherein a projection of the second extension on the first surface of the circuit board encloses a second end of the slit in the plane of the first surface; and a third extension portion at least a portion of which is positioned over the conductive layer on the second side of the slit extending from an end of the second extension portion in parallel with the slit; and wherein the feeding line is arranged to receive a transmission signal on the first side of the slit such that the transmission signal is delivered to the first extension portion through a path via the conductive layer and the connection terminal on the second side of the slit.