A preliminary image is obtained by irradiating illumination light, whose light quantity distribution is set at a standard state, to a specimen obtained by injecting fluid into a well from an illuminator capable of arbitrarily setting the light quantity distribution, for example, by a liquid crystal shutter, and a luminance distribution of the preliminary image is detected. As a result, luminance nonuniformity in the preliminary image is canceled by performing imaging with a light quantity distribution of illumination light to be incident on the well set such that an incident light amount is larger in a part with lower luminance.