An RF tag antenna (10, 11), comprising: a first insulating substrate having a first principal surface and a second principal surface that is opposite to the first principal surface; a first waveguide device provided on the first principal surface; a second waveguide device provided on the second principal surface; a power feeding part that is provided on a side surface of the first insulating substrate and electrically connected to the second waveguide device at one end thereof; and a short-circuit part that is provided on the side surface of the first insulating substrate and electrically connected to the first waveguide device at one end thereof and to the second waveguide device at another end thereof, wherein the first insulating substrate, the first waveguide device, the second waveguide device, the power feeding part and the short-circuit part form a plate-shaped inverted-F antenna that receives a radio wave transmitted from a reader, and an inductor pattern formed by the first waveguide device, the short-circuit part, the second waveguide device and the power feeding part and a first capacitor formed by the first waveguide device, the second waveguide device and the first insulating substrate form a resonant circuit that resonates in a frequency band of the radio wave, characterized in that the RF tag antenna further comprises an extension part that extends from a side edge of any one waveguide device of the first waveguide device and the second waveguide device and is stuck to the other waveguide device with a second insulating substrate interposed therebetween such that a second capacitor is formed by the extension part, the second insulating substrate, and the other waveguide device of the first waveguide device and the second waveguide device. a first insulating substrate having a first principal surface and a second principal surface that is opposite to the first principal surface; a first waveguide device provided on the first principal surface; a second waveguide device provided on the second principal surface; a power feeding part that is provided on a side surface of the first insulating substrate and electrically connected to the second waveguide device at one end thereof; and a short-circuit part that is provided on the side surface of the first insulating substrate and electrically connected to the first waveguide device at one end thereof and to the second waveguide device at another end thereof, wherein the first insulating substrate, the first waveguide device, the second waveguide device, the power feeding part and the short-circuit part form a plate-shaped inverted-F antenna that receives a radio wave transmitted from a reader, and an inductor pattern formed by the first waveguide device, the short-circuit part, the second waveguide device and the power feeding part and a first capacitor formed by the first waveguide device, the second waveguide device and the first insulating substrate form a resonant circuit that resonates in a frequency band of the radio wave, characterized in that the RF tag antenna further comprises an extension part that extends from a side edge of any one waveguide device of the first waveguide device and the second waveguide device and is stuck to the other waveguide device with a second insulating substrate interposed therebetween such that a second capacitor is formed by the extension part, the second insulating substrate, and the other waveguide device of the first waveguide device and the second waveguide device.