A contactless semiconductor device is provided where a low power consumption is achieved and wireless communication is carried out stably. A power supply level of a reader/writer and the operating state of internal blocks of an ID chip are monitored. In accordance with such conditions, a clock signal of an optimal frequency and a power supply potential of an optimal level are selected to be supplied. By setting the operating mode in accordance with the power supply level or the power saving mode in accordance with the operating state of each block, a low power consumption ID chip and stable wireless communication can be provided.