In order to reduce power consumption, a semiconductor device includes an RTC for generating a piece of time information and a first activation signal SW3, a comparator for determining whether the value of an analog input signal exists within a predetermined range, an AD conversion circuit for converting the analog input signal to a digital signal in response to a common activation signal, and a CPU for processing the digital signal in response to the common activation signal. When the analog input signal does not exist within the predetermined rang, the comparator generates the common activation signal. Then, the CPU stores the piece of digital information corresponding to the digital signal as well as the piece of time information from the RTC into a storage circuit.