PURPOSE: To maintain the number of pictures recorded on a recording medium always constant by performing temporary quantization using an arbitrary quantization matrix and deciding the quantization matrix by which the most adequate compressing ratio to obtain a prescribed data quantity can be obtained from the temporary compressing ratio.

CONSTITUTION: The image of an object photographed by an image pickup section 1 is inputted to an A/D-converting section 2 as video signals and sent to a coding section 4 after the signals are converted into digital picture data. At the coding section 4, the inputted picture data are divided into plural blocks, each of which is composed of (n×n) picture elements, by means of a discrete cosine transforming(DCT) section 4a and the data are coded by performing two-dimensional DCT at every block. Then the coded data are inputted to a data quantity measuring section 5 and measurement is made to confirm whether or not the quantity of all compression-coded data is smaller than a prescribed data quantity. When the all data quantity exceeds the prescribed quantity, a compressing ratio calculating section 6 calculates the compressing ratio at which the quantity of all compressed data becomes smaller than the prescribed quantity and a quantizing section 4b again makes quantization of the DCT coefficient by selecting another quantization matrix having a threshold which different from that of the preceding quantization matrix.