A main memory stores M-bit gradation image data (M is a natural number). A correction function generating unit generates a correction function in N-bit gradation (N is a natural number, N>M) from image data. A display image generating unit generates display image data using a correction function generated by the correction function generating unit. The display image generating unit uses a correction function so as to generate display image data resulting from correcting image data obtained by decoding compressed image data in M-bit gradation with the restriction of M-bit quantization.