An A/D converter of an successive approximation type according to the present invention comprises a sample hold circuit, a reference voltage generating circuit, a comparator for comparing the reference voltage generated by the reference voltage generating circuit to a value of the input analog signal retained in the sample hold circuit, a control circuit for successively controlling the reference voltage generating circuit so that a value of the reference voltage approximates to the value of the input analog signal retained in the sample hold circuit, a buffering circuit for outputting an output value corresponding to an output voltage of the comparator, a latch circuit for retaining the output value of the buffering circuit corresponding to the output value of the comparator per bit as a digital value, and a buffering control circuit for blocking a power supply to the buffering circuit during the sampling period is provided.