A tone adjustment unit assumes that the following relationship is satisfied: LO=DO+Da=TD(I)+Da=TL(I), in which I represents a luminance value normalized to have a maximum value of one in first image data, DO represents a luminance value normalized to have a maximum value of one in second image data, Da represents a luminance value under ambient light during viewing, LO represents a luminance value of the second image data while taking into consideration effects of the ambient light during viewing, TD(I) represents a tone curve used for conversion, and TL(I) represents a tone curve taking into consideration the environment. The tone curve TD(I) is adjusted so that a function representing an inclination γ(I) after converting LO=TL(I) to a double logarithmic plot is non-zero over an entire dynamic range.