PURPOSE: To reduce deterioration in picture quality by measuring a generated code quantity for each block and controlling a round-off range of picture information based on the result of measurement thereby enabling the encoding and decoding of picture data.

CONSTITUTION: A DCT coefficient as a result of discrete cosine transformation at an orthogonal transformation device 3 is quantized by a quantizer 4, subject to variable length coding at a variable length encoder 5 and outputted to a storage medium of picture data. In this case, a code quantity measurement device 6 measures the code quantity of data subject to variable length coding and gives the result of measurement to a coefficient transformation section 7. The coefficient transformation section 7 compares each picture code quantity sent from the code quantity measurement device 6 with a predetermined code quantity latched in an object setting section 8. Then a coefficient transformation section 7 corrects the code quantity increasingly when the measured code quantity is lower and corrects the code quantity decreasingly when the measured code quantity is in excess.