An RF tag circuit connected to an antenna and a load is provided. The RF tag circuit includes: a rectification circuit rectifying a radio wave received by the antenna and supplying DC power; a matching circuit having a changeable impedance and disposed between the antenna and the rectification circuit; a control part repeatedly controlling activation and stopping of the load; and an adjustment part changing the impedance of the matching circuit in a predetermined direction, storing a first electric power generated by the rectification circuit when a predetermined time has elapsed after the load is activated, and changing the impedance of the matching circuit based on a magnitude relationship between a second electric power generated by the rectification circuit when the predetermined time has elapsed after the load is activated at a timing after the first electric power is generated and the stored first electric power.