1. An image sensing apparatus comprising: an image sensing device that has a plurality of pixels; a smoothing unit that smoothes a first dark image signal acquired with said image sensing device shielded from light and generates a second dark image signal; a subtraction unit that subtracts the second dark image signal from the first dark image signal; an extraction unit that extracts, as a defective pixel of said image sensing device, a pixel of which an obtained difference is outside a preset range; and a correction unit that corrects, out of subject image signals output from said image sensing device, an image signal of the defective pixel extracted by said extracting unit, using an image signal of a pixel peripheral to the defective pixel.