A semiconductor integrated circuit is operative in a plurality of different modes. A plurality of select signals whose number corresponds to modes selected from a plurality of different modes are outputted. In response to the select signals, it is detected whether at least two operation modes are selected simultaneously. If so, a detection signal is outputted. In response to this detection signal, the operation of the semiconductor integrated circuit is stopped. Further, in response to the select signal, the semiconductor integrated circuit is activated in a mode by means of a predetermined select signal of these select signals. Further, in response to these select signals, the selected mode can be detected.