A semiconductor device is provided with a power supply circuit having a function to generate a power supply voltage from a wireless signal and an A/D converter circuit having a function to detect the strength of the wireless signal by an A/D conversion of a voltage generated from the wireless signal. This enables to provide a semiconductor device which does not require replacement of batteries, has few limitations on its physical shape and mass, and has a function to detect a physical position. By formation of the semiconductor device with use of a thin film transistor formed over a plastic substrate, a lightweight semiconductor device, which has flexibility in physical shape and a function to detect a physical location, can be provided at low cost.