In a case where an ASK method is used for a communication method between a semiconductor device and a reader/writer, the amplitude of a radio signal is changed by data transmitted from the semiconductor device to the reader/writer when data is not transmitted from the reader/writer to the semiconductor device. Therefore, in some cases, the semiconductor device mistakes data transmitted from the semiconductor device itself for data transmitted from the reader/writer to the semiconductor device. The semiconductor device includes an antenna circuit, a transmission circuit, a reception circuit, and an arithmetic processing circuit. The antenna circuit transmits and receives a radio signal. The transmission circuit outputs to the reception circuit a signal showing whether or not the antenna circuit is transmitting the radio signal.