To accelerate image compression by counting the number of invalid coefficients in discrete cosine transform(DCT) coefficients provided by performing DCT to image data, providing the predictive code amount of compressed data, while referring to a code amount prediction table, and deriving an optimum control coefficient.

At a code amount predictive part M1, two-dimensional DCT processing is performed to the pixel block of image data extracted at a blocking part 20 by a DCT part 21. The coefficient provided by DCT processing is quantized at a quantizing part 22. At an invalid coefficient count part 23, the number of invalid coefficients in the coefficients provided after quantization is counted. At a code amount estimating part 24, while referring to a code amount predictive table TB1 expressing the correspondent relation between the number of invalid coefficients in the pixel block and the amount of codes in the pixel block, a predictive code amount is provided from the number of invalid coefficients. The provided predictive code amount is applied to an optimum control coefficient deriving part.