1. An image processing apparatus comprising: a generation unit configured to generate a background model of a multinomial distribution from an acquired image, and generate a background image using the background model; a moving object detection unit configured to detect a moving object in the acquired image, wherein the moving object is an area in the acquired image different from the background image; and a reliability processing unit configured to calculate a value for a pixel of the acquired image indicating a probability of the pixel being a non-moving object using the acquired image, the background image and a temporarily paused image, wherein the temporarily paused image comprises the moving object which is paused for at least a predetermined time period, wherein the background model includes a distribution in which a luminance value at a predetermined coordinate is associated with an appearance frequency of the luminance value, and wherein a determination unit determines the predetermined coordinate as a processable area when the distribution with highest appearance frequency among distributions in the multinomial distribution is larger than a predetermined value, wherein the predetermined value is calculated from a number of terms of the multinomial distribution and a number of updates of the background model.