A data conversion device is provided with a data converting means that sequentially converts first data into second data of the number of second bits, wherein an analog signal is quantized into the first data by the number of first bits, and the first and second data can be first and second maximum values, respectively. The data converting means is comprised of a first conversion means (steps 21 and 23) that, when a value of the fast data is not the first maximum value, converts the first data to the second data by adding 0 to a lower bit side of the first data and a second conversion means (steps 21 and 24-26) that converts the first data to the second data so that, when a value of the first data is the fast maximum value, a value can be made larger, in accordance with a value be-fore or after the first data, than the data of the number of second bits obtained by adding 0 to the lower bit side of the first data. With the structure, when the data obtained by quantizing an analog signal is converted to data with the number of more bits, a rounding error by quantizing is improved as much as possible.