An antenna capable of receiving circularly polarized waves and performing impedance matching between the antenna and an IC (integrated circuit) of a semiconductor device, and a semiconductor device having such an antenna. The antenna has a first conductor pattern with a loop configuration having a cut section, a second conductor pattern, a third conductor pattern, and a feeding section. A first end portion of the second conductor pattern and a first end portion of the third conductor pattern are connected to the first conductor pattern. A second end portion of the second conductor pattern and a second end portion of the third conductor pattern are connected to the feeding section. The total length of the second conductor pattern is longer than the total length of the third conductor pattern, and the second conductor pattern is placed closer to the cut section than the third conductor pattern is.