A semiconductor device for an IC tag comprises a receive unit for demodulating receive data from a received RF signal and a signal processing unit for detecting a command comprising data signals from the receive data demodulated by the receive unit and executing processing based on the command, wherein the signal processing unit has a command acceptance mode for detecting the command from the receive data and a command execution mode for executing the command, and generates an error signal when the data signal is not detected from the receive data in the command acceptance mode.