To provide a reader/writer antenna 100 which is installed having a gate shape and communicates with an RFID tag, which prevents the occurence of a non-sensible zone or easily deals with an area of the non-sensible zone, has a small width, and is stable against mechanical impact.

A dipole antenna 1, arranged parallel with a center axis, and a rectangular reflection plate 2, which faces the dipole antenna 1 and is elongated along the center axis, are provided in a cylindrical radome 3. The die pole antenna 1 is positioned in the center of the reflection plate 2.