To solve the problem that the conventional moving picture coding/ decoding system provides 31 types of quantizers QP, however, sets the quantizing step width Q being a factor for determining the quantizing accuracy at equal spacings (Q=2×QP, Q=1 to 31) to result in a slow change of picture quality, relative to the change of the QP, and hence a great change of picture quality due to the change of the QP, if small QP.

The picture coding method including an N×N two-dimensional DCT, quantizing of the DCT coefficient and coding of the quantized DCT coefficient uses a DCT coefficient quantizing method, such that the input to a quantizing process takes a value of the DCT coefficient multiplied by N, and the DCT coefficient multiplied by N is quantized. The picture decoding method including decoding the quantized DCT coefficient, inverse quantizing of the decoded quantized DCT coefficient and the N×N two-dimensional inverse DCT uses an inverse quantizing-inverse DCT method, such that the output of an inverse quantizing process takes a DCT coefficient multiplied by N, and rules are defined for restoring a DCT coefficient at a specified accuracy from the DCT coefficient multiplied by N in the reserve DCT process.