1. An imaging apparatus comprising: an image pickup element configured to convert light from an optical system for a short time into an electric signal to output a short-exposure-image data, and convert light from the optical system for a long time into an electric signal to output a long-exposure-image data; a white balance evaluation value acquiring device configured to acquire a white balance evaluation value for short-exposure-image data from the short-exposure-image data, and acquire a white balance evaluation value for long-exposure-image data from the long-exposure-image data; a white extraction range setting device configured to set a white extraction range for short-exposure-image data and a white extraction range for long-exposure-image data; a white extraction device configured to extract a short-exposure-image data white extraction result on the basis of the white balance evaluation value for short-exposure-image data and the white extraction range for short-exposure-image data, and extract a long-exposure-image data white extraction result on the basis of the white balance evaluation value for long-exposure-image data and the white extraction range for long-exposure-image data; a correction factor calculating device configured to calculate a short exposure image white balance correction factor by using the short-exposure-image data white extraction result, and calculate a long exposure image white balance correction factor by using the long-exposure-image data white extraction result; a white balance correcting device configured to correct the short-exposure-image data to form white balance corrected short-exposure-image data by using the short exposure image white balance correction factor, and correct the long-exposure-image data to form white balance corrected long-exposure-image data by using the long exposure image white balance correction factor; and a synthesizing device configured to synthesize the white balance corrected short-exposure-image data and the white balance corrected long-exposure-image data.