An antenna (A1, A2, A3) comprising antenna elements (a2, a2-1, a2-2, a2-3, a2-4) provided on sides of a nearly rectangular shaped conductive board, respectively, wherein: each of the antenna elements comprises a feeding wire (a23, a23-1, a23-2, a23-3, a23-4) and a split-ring conductor (a22, a22-1, a22-2, a22-3, a22-4) of a shape which is a ring but is partially cut by a split portion (a21, a21-1, a21-2, a21-3, a21-4); and the feeding wire is electrically connected with the split-ring conductor and extends in a direction which traverses a region (a24, a24-1, a24-2, a24-3, a24-4) formed inside the split-ring conductor, and among four of the antenna elements (a2-1, a2-2, a2-3, a2-4), two (a2-1 and a2-3) of the antenna elements which are arranged on two of the sides of the conductive board opposite to each other and are not arranged on two of the sides of the conductive board adjacent to each other are configured to be fed by one feeding point via the respective feeding wire, and remaining two (a2-2 and a2-4) of the antenna elements which are arranged on remaining two of the sides of the conductive board opposite to each other are configured to be fed by another feeding point via the respective feeding wire, so that orientations of electric fields in polarization directions of the antenna elements arranged on any two of the sides of the conductive board opposite to each other are substantially same as each other, and one of the two of the antenna elements arranged on any two of the sides of the conductive board opposite to each other is configured to be simply fed by one of the feeding points, while a remaining one of the two of the antenna elements is configured to be fed by the one of the feeding points via a 180 degrees phase shifter. each of the antenna elements comprises a feeding wire (a23, a23-1, a23-2, a23-3, a23-4) and a split-ring conductor (a22, a22-1, a22-2, a22-3, a22-4) of a shape which is a ring but is partially cut by a split portion (a21, a21-1, a21-2, a21-3, a21-4); and the feeding wire is electrically connected with the split-ring conductor and extends in a direction which traverses a region (a24, a24-1, a24-2, a24-3, a24-4) formed inside the split-ring conductor, and among four of the antenna elements (a2-1, a2-2, a2-3, a2-4), two (a2-1 and a2-3) of the antenna elements which are arranged on two of the sides of the conductive board opposite to each other and are not arranged on two of the sides of the conductive board adjacent to each other are configured to be fed by one feeding point via the respective feeding wire, and remaining two (a2-2 and a2-4) of the antenna elements which are arranged on remaining two of the sides of the conductive board opposite to each other are configured to be fed by another feeding point via the respective feeding wire, so that orientations of electric fields in polarization directions of the antenna elements arranged on any two of the sides of the conductive board opposite to each other are substantially same as each other, and one of the two of the antenna elements arranged on any two of the sides of the conductive board opposite to each other is configured to be simply fed by one of the feeding points, while a remaining one of the two of the antenna elements is configured to be fed by the one of the feeding points via a 180 degrees phase shifter.