1. A moving image noise reduction processing apparatus for reducing noise in a digitalized time-series image signal, comprising: a spatial noise reduction unit applying spatial noise reduction processing to the image signal; an image signal storage unit storing an image signal to which at least the spatial noise reduction processing has been applied; and a temporal noise reduction unit performing temporal noise reduction processing based on an image signal to be processed to which the spatial noise reduction processing has been applied in the spatial noise reduction unit and an image signal which is stored in the image signal storage unit and is earlier than the image signal to be processed; wherein the temporal noise reduction unit comprises: a temporal noise amount detection unit detecting an amount of temporal noise in a pixel of interest in an image signal to be processed to which the spatial noise reduction processing has been applied, by using the previous image signal based on a temporal noise model; and a temporal noise removal processing unit applying noise removal processing to a pixel of interest in an image signal to be processed to which the spatial noise reduction processing has been applied, based on the amount of temporal noise detected in the temporal noise amount detection unit; and wherein: the digitalized time-series image signal is obtained by picking up an image by an image sensor at a predetermined frame rate, amplifying by a predetermined gain, and being digitalized; the temporal noise amount detection unit comprises a representative value estimation unit estimating a representative value of the pixel of interest based on an image signal to be processed to which the spatial noise reduction processing has been applied and at least one of one or more previous image signals stored in the image signal storage unit; the temporal noise model is generated by modeling a relationship between the representative value and an amount of noise and is dependent on at least one of items of information indicating a temperature of the image sensor at the time when the image signal has been picked up, a gain for the image signal, and a frame rate of the time-series image signal; and the temporal noise amount detection unit further comprises a temporal noise amount calculation unit calculating an amount of temporal noise based on the temporal noise model by using the representative value of the pixel of interest estimated in the representative value estimation unit and at least one of items of information indicating a temperature of the image sensor at the time when the image signal has been picked up, a gain for the image signal, and a frame rate of the time-series image signal.