In an image synthesizer (1), code stream analyzers (10, 11), code block extraction units (12, 13) and EBCOT decoders (14, 15) work together to decode encoded code streams (D10, D11) encoded according to the MPEG-2000 Standard and generate quantization coefficients (D16, D17) for each code block. In a cross-fading unit (16), multipliers (17, 18) multiply the quantization coefficients (D16, D17) by coefficients (.alpha.(t), (1-.alpha.(t))) and an adder (19) adds together the results of multiplication to provide a cross-fading quantization coefficient (D20). An EBCOT encoder (20), rate controller (21) and code stream generator (22) work together to encode the cross-fading quantization coefficient (D20) to provide a final encoded code stream (D23). Therefore, the image synthesizer (1) can combine two encoded code streams easily and effectively with a reduced use of a memory capacity.