To predict a target code amount from a code amount calculated on a single quantizer scale. The present invention stores relationship information on a plurality of pieces of quantized data for prediction having different MB generated code amounts on a fixed quantizer scale (QI) as prediction curves, the relationship information indicating a relationship between a plurality of quantizer scales (Q) representing a plurality of quantizer steps and MB generated code amounts. The present invention quantizes image data on a macroblock-by-macroblock basis on the single fixed quantizer scale (QI) out of the predetermined plurality of quantizer scales (Q), thereby generating temporary quantized data, and calculates the MB generated code amount of each macroblock unit of this temporary quantized data. The present invention then selects a piece of quantized data for prediction from the plurality of prediction curves based on the MB generated code amount of the temporary quantized data and the fixed quantizer scale (QI), and predicts the MB generated code amounts for situations when the image data is quantized on quantizer scales (Q) not selected by a quantization selection section, based on the selected piece of quantized data for prediction.