1. An image processing apparatus that tone-compresses input luminance values representing luminance values of an input image, the image processing apparatus comprising: conversion-curve calculating means for calculating a conversion curve that is used for tone-compressing luminance values, on the basis of a distribution of the input luminance values; global-luminance calculating means for calculating global luminance values representing luminance values of a global-luminance image composed of low-frequency components of the input image; tone compressing means for tone-compressing the input luminance values and the global luminance values according to the conversion curve; and contrast correcting means for correcting contrast of a tone-compressed input image composed of the tone-compressed input luminance values, on the basis of a slope of the conversion curve and the tone-compressed global luminance values.