1. An image combining device comprising: a motion vector calculating portion calculating a motion vector between different divided exposure images of a plurality of divided exposure images obtained as a result of continuous shooting, wherein the image combining device positions the divided exposure images based on the motion vector thus calculated, and combines the divided exposure images together one on top of another so as to form one composite image, wherein the image combining device further comprises: a mixture proportion calculating portion calculating, based on information commensurate with a motion of an image between the different divided exposure images, a mixture proportion of each divided exposure image to the composite image, wherein the image combining device combines the divided exposure images together one on top of another in mixture proportions thus calculated so as to form the composite image.