1. An image composition method comprising: composing a high-sensitivity image signal H and a low-sensitivity image signal L, so as to generate a composite image signal Gc according to a composing expression: 
 Gc=k·H+ (1− k )· L where k is a composite parameter satisfying 0≦k≦1, wherein the high-sensitivity image signal H is generated based on high-sensitivity image outputs from corresponding ones of a plurality of pixel positions in a solid-state imaging element that has imaged a subject; the low-sensitivity image signal L is generated based on low-sensitivity image outputs from corresponding ones of the plurality of pixel positions in the solid-state imaging element; and the composing is performed at every common pixel position by determining k at every common pixel position.