An external input operation clock signal is required for operating a conventional A/D converter, and the performance of the A/D converter is undesirably determined by the characteristics of the external input operation clock. A clock generator circuit for automatically generating an operation clock is provided inside an A/D converter to make the A/D converter require no external input operation clock. Further, a circuit for detecting the operation times of the constituents of the A/D converter is provided to generate a clock with which the A/D converter is optimally operated, thereby realizing high-speed operation and low power consumption.