In a converter which operates in a serial-parallel manner and performs analog-to-digital (A/D) conversion, the number of high bits is made more than half the entire number of bits, with the number of low bits is given by half the entire number of bits, thereby acquiring high-bit and low-bit data. When no match exists between the high-bit data and the low-bit data, the high-bit data can be modified by means of the low-bit data.