A multiband antenna (100H, 100N) which is configured to be connected to a host conductor when used, wherein: the multiband antenna (100H, 100N) extends longer in a first direction than in a second direction perpendicular to the first direction; the multiband antenna (100H, 100N) has a conductor main portion and a ground terminal; the conductor main portion extends in a horizontal plane which is defined by the first direction and the second direction; the conductor main portion is formed with an opening and a slot; the slot extends longer in the first direction than in the second direction; the conductor main portion has a first short edge, a second short edge, a first long edge and a second long edge; the first short edge and the second short edge are positioned at opposite ends, respectively, of the conductor main portion in the first direction; the first long edge and the second long edge are positioned at opposite ends, respectively, of the conductor main portion in the second direction; the opening is formed at the first short edge and connects the slot with an outside of the conductor main portion in the first direction; the ground terminal is configured to be connected to the host conductor when the multiband antenna (100H, 100N) is used; the ground terminal extends from the second long edge; and the ground terminal is positioned closer to the first short edge than to the second short edge in the first direction, wherein the multiband antenna (100H, 100N) further comprises an additional conductor portion, wherein the additional conductor portion has a first portion and a second portion; the first portion extends from the conductor main portion so as to be apart from the slot in the second direction; the first portion has a first length in the second direction; the second portion extends in the first direction from the first portion; the second portion has a second length in the first direction; and the second length is greater than the first length, wherein the additional conductor portion further has a folded portion; and the folded portion extends from the second portion in a direction which intersects with the horizontal plane, wherein the additional conductor portion further has an additional extending portion; the additional extending portion extends from the folded portion in a direction which intersects with the folded portion; and further comprising a high frequency power source, characterized in that the high frequency power source is configured to feed the multiband antenna (100H, 100N) across the slot. the multiband antenna (100H, 100N) extends longer in a first direction than in a second direction perpendicular to the first direction; the multiband antenna (100H, 100N) has a conductor main portion and a ground terminal; the conductor main portion extends in a horizontal plane which is defined by the first direction and the second direction; the conductor main portion is formed with an opening and a slot; the slot extends longer in the first direction than in the second direction; the conductor main portion has a first short edge, a second short edge, a first long edge and a second long edge; the first short edge and the second short edge are positioned at opposite ends, respectively, of the conductor main portion in the first direction; the first long edge and the second long edge are positioned at opposite ends, respectively, of the conductor main portion in the second direction; the opening is formed at the first short edge and connects the slot with an outside of the conductor main portion in the first direction; the ground terminal is configured to be connected to the host conductor when the multiband antenna (100H, 100N) is used; the ground terminal extends from the second long edge; and the ground terminal is positioned closer to the first short edge than to the second short edge in the first direction, wherein the multiband antenna (100H, 100N) further comprises an additional conductor portion, wherein the additional conductor portion has a first portion and a second portion; the first portion extends from the conductor main portion so as to be apart from the slot in the second direction; the first portion has a first length in the second direction; the second portion extends in the first direction from the first portion; the second portion has a second length in the first direction; and the second length is greater than the first length, wherein the additional conductor portion further has a folded portion; and the folded portion extends from the second portion in a direction which intersects with the horizontal plane, wherein the additional conductor portion further has an additional extending portion; the additional extending portion extends from the folded portion in a direction which intersects with the folded portion; and further comprising a high frequency power source, characterized in that the high frequency power source is configured to feed the multiband antenna (100H, 100N) across the slot.