A control circuit controls first and second clock generator subcircuits so that one subcircuit of the first and second clock generator subcircuits operates for a comparison voltage generating interval, then another subcircuit operates for a clock generating interval, and so that the first and second clock generator subcircuits alternately repeat processes of the comparison voltage generating interval and the clock generating interval. For the comparison voltage generating interval, each of the first and second clock generator subcircuits is controlled to generate a comparison voltage and output the same voltage to an inverted output terminal of a comparator. For the clock generating interval, each of the first and second clock generator subcircuits compares an output voltage from a current-voltage converter circuit with the comparison voltage.