1. An imaging device comprising: an imaging unit that images a subject with three or more kinds of exposures, ratios of respective pairs of exposures of which adjacent to each other in an ascending order or a descending order are equal; a combining unit that combines, on the basis of the ratios, pixel signals corresponding to the respective three or more kinds of exposures obtained by imaging the subject with the imaging unit; and an exposure adjusting unit that adjusts, on the basis of information concerning brightness of an imaging environment, while keeping the ratios of the respective pairs of exposures adjacent to each other equal, a control amount for an exposure of the imaging unit such that the three or more kinds of exposures are exposures suitable for the brightness of the imaging environment.