An image-data-distribution-model updating apparatus includes: a determining unit that determines, concerning image data continuous in time series, to which of plural distribution models indicating an average of signal levels for each of plural color components, variance of a signal level distribution of all the plural color components, and reliability of all the plural color components the target image data belongs; and an updating unit that updates the average, the variance, and the reliability of a distribution model having the highest reliability among the distribution models to which it is determined that the target image data belongs and updates the reliability of the other distribution models.