An antenna for a RFID writer/reader includes a substrate made of a dielectric substance, and a conductor pattern formed on the substrate and including a feeding point, an open end, an extension part extending from the feeding point, and a spiral part extending spirally from an opposite end of the extension part to the open end. The spiral part includes a part juxtaposed with the extension part. Adistance along length directions of the conductor pattern, from a zero point at which a current is zero in the part upon feeding power to the feeding point, to a point at which a line perpendicular to the part and passing through the zero point intersects the extension part, is set to a second distance at which an electric field strength generated as a combination of electric fields generated at these points upon the feeding of power enables communications with the wireless tag.