In a quantizing circuit 15, DCT coefficients output from a DCT circuit 14 are quantized with the quantizing step corresponding to a quantizing index supplied from a quantizing index determining circuit 25. In the quantizing index determining circuit 25, the quantizing index is determined on the basis of a first variable corresponding to an occurrence code amount as a data accumulative amount of a buffer 17 and a second variable corresponding to the activity of macroblocks output from an activity detection circuit 24. In this case, the quantizing index determining circuit 25 adaptively sets a fixing mode for fixing the first variable or a variable mode for making variable the