1. An image pickup device comprising: an image pickup unit which picks up a subject at first and second exposure times; a high dynamic range (HDR) image data generating unit which generates first HDR image data configured by pixel data of which pixel values are expressed by a floating-point form in which a mantissa part has N bits (N is a natural number of 2 or more), a cardinal number is X (X is a natural number of 2 or more) and an exponential part of the cardinal number has M bits (M is a natural number of 2 or more), on the basis of first image data obtained by picking up the subject at the first exposure time and second image data obtained by picking up the subject at the second exposure time; an image processing unit which performs a predetermined image process including a floating-point arithmetic operation process, with respect to the first HDR image data generated by the HDR image data generating unit; and a HDR image data converting unit which converts the first HDR image data after the image process into second HDR image data configured by pixel data of which the pixel values are expressed by a floating-point form in which the mantissa part has n bits (n is a natural number of N>n) and the exponential part of the cardinal number has m bits (m is a natural number of M>m).