1. A digital camera having a focusing function and a focal length varying function of a main lens, the digital camera comprising: a main imaging unit configured to generate a main image by performing a photoelectric conversion from a subject obtained via the main lens; a sub-imaging unit configured to generate a sub-image by performing a photoelectric conversion from the subject obtained via a sub-lens; a focal length acquiring unit configured to acquire the focal length of the main lens; a magnification setting unit configured to set a level of magnification on the basis of the focal length; a composite image generating unit configured to generate a composite image by combining the main image and the sub-image, after at least one of the main image and the sub-image is magnified at the level of magnification; and an image display unit configured to display the composite image.