1. An image processing apparatus comprising: an image-capturing-mode selecting portion configured to select one image-capturing mode from a plurality of image-capturing modes; a signal-value-region setting portion configured to set a predetermined signal-value region on image signals input from an image-acquisition element; a noise-level calculating portion configured to calculate noise levels; a noise-reduction processing portion configured to perform noise reduction processing on the input image signals based on the noise levels calculated by the noise-level calculating portion; and a reference-noise-model storing portion configured to store a plurality of reference noise models determined in accordance with properties of the image-acquisition element; wherein: each reference noise model indicates a relationship between signal value and noise level, and has a characteristic such that the noise level increases with an increase in the signal value; the signal-value-region setting portion is configured to set a signal-value region as the predetermined signal-value region, signal values of the signal-value region being at or below a threshold from among a plurality of thresholds determined in association with the image-capturing modes, when the image-capturing mode selected by the image-capturing-mode selecting portion is a specific image-capturing mode; the noise-level calculating portion is configured to calculate (i) a noise level of a signal-value region other than the predetermined signal-value region, based on a reference noise model, and (ii) a noise level of the predetermined signal-value region, based on an applied noise model, the applied noise model being modified so that the noise level of the predetermined signal-value region is higher than the noise level of the reference noise model which corresponds to the image-capturing mode selected by the image-capturing-mode selecting portion, when the predetermined signal-value region is set by the signal-value-region setting portion; and the noise-level calculating portion is configured to calculate a noise level of an entire region of the input image signals based on the reference noise model, when the predetermined signal-value region is not set by the signal-value-region setting portion.