An RFID tag communication apparatus comprising: a housing constituting an equipment shell including a bottom surface in contact with a predetermined installation surface; an antenna provided to extend between distances h1 and h2 from said bottom surface that transmits and receives information by wireless communication with an RFID tag including an IC circuit part that stores information and a tag antenna that transmits and receives information; and a radio frequency circuit including carrier wave transmitting means for generating a carrier wave of a frequency fc for supplying power to said RFID tag, for transmitting and receiving information with said RFID tag; characterized in that the frequency fc is set within the range fo<fc<fm, in accordance with the extension of the antenna from the bottom surface between h1 and h2 under the conditions that: fm is set for a resonance frequency of said antenna when said bottom surface is fixed in contact with said installation surface made of a metal, and fo is set for a resonance frequency of said antenna when said bottom surface is fixed in contact with said installation surface made of a non-metal or said bottom surface is separated from said installation surface thereby providing a single unit state of said RFID tag communication apparatus, and said resonance frequency fm and a frequency range length |fm - fo| between fm and fo decrease with increasing distances h1 and h2 from said bottom surface. a housing constituting an equipment shell including a bottom surface in contact with a predetermined installation surface; an antenna provided to extend between distances h1 and h2 from said bottom surface that transmits and receives information by wireless communication with an RFID tag including an IC circuit part that stores information and a tag antenna that transmits and receives information; and a radio frequency circuit including carrier wave transmitting means for generating a carrier wave of a frequency fc for supplying power to said RFID tag, for transmitting and receiving information with said RFID tag; characterized in that the frequency fc is set within the range fo<fc<fm, in accordance with the extension of the antenna from the bottom surface between h1 and h2 under the conditions that: fm is set for a resonance frequency of said antenna when said bottom surface is fixed in contact with said installation surface made of a metal, and fo is set for a resonance frequency of said antenna when said bottom surface is fixed in contact with said installation surface made of a non-metal or said bottom surface is separated from said installation surface thereby providing a single unit state of said RFID tag communication apparatus, and said resonance frequency fm and a frequency range length |fm - fo| between fm and fo decrease with increasing distances h1 and h2 from said bottom surface. fm is set for a resonance frequency of said antenna when said bottom surface is fixed in contact with said installation surface made of a metal, and fo is set for a resonance frequency of said antenna when said bottom surface is fixed in contact with said installation surface made of a non-metal or said bottom surface is separated from said installation surface thereby providing a single unit state of said RFID tag communication apparatus, and said resonance frequency fm and a frequency range length |fm - fo| between fm and fo decrease with increasing distances h1 and h2 from said bottom surface.