An RFID tag substrate on which an RFID tag that performs wireless communication with a reader/writer is disposed includes a substrate layer, and a functional layer laminated or stacked on the substrate layer. The functional layer includes a plurality of layers laminated together including a high-dielectric-constant layer having a predetermined dielectric constant, and a high-permeability layer having a predetermined permeability. A product of the predetermined dielectric constant and the predetermined permeability is not smaller than 250.