PURPOSE: To provide an image data processor and a quantizing method capable of securing satisfactory picture quality by reducing block distortion.

CONSTITUTION: Image data stored in an image data memory device 2 is discrete cosine-transformed by a discrete cosine transformer 3, and a bias is added on the DC component of data after receiving discrete cosine transformation by a bias adder 10. The cumulation of the relative error of the DC component after quantization and the one before quantization for plural blocks is calculated by a cumulative error calculation device 11 as executing the quantization processing of a bias-added value by a DC quantizer 13, and the encoding processing of quantize data with minimum cumulative error calculated by the device 11 is executed by an encoder 5.