1. A density adjustment method for adjusting a density of a diagnostic image formed on a film, comprising: exposing an image to form an latent image on the film based on test exposure data and/or diagnostic image data; developing the latent image to form a developed image; measuring a density of the developed image; creating a lookup table for relating the diagnostic image data and amount of exposure so as to reproduce a density specified by the diagnostic image data, based on the test exposure data and a measured density of the developed image formed on the film by the test exposure data; and compensating by correcting at least one of an exposure condition in the exposing step and a development condition in the developing step to ensure that the next film has the optimized density, based on the difference between the measured density value obtained by exposing a partial area of the film with a prescribed exposure amount at the time of forming the diagnostic image with the diagnostic image data and by measuring the density of the partial area of the film, and a density value for comparison corresponding to the prescribed exposure amount; wherein, a density of a prescribed area, exposed by a specific exposure amount, in an image formed by an exposure based on the test exposure data, is measured, and the measured value of the density is used as the density value for comparison; at the same time, the specific exposure amount is used as the prescribed exposure amount for exposing the partial area of the film at the time of forming a diagnostic image.