A culture vessel that houses a culture liquid and a specimen is irradiated with pattern light having a pattern that is previously set for the culture vessel. Transmitted light transmitted through the culture liquid in the culture vessel because of the irradiation with the pattern light is detected. Optical characteristics of an adjusting optical system that adjusts refraction of light caused by the shape of the liquid surface of the culture liquid in the culture vessel are adjusted, on the basis of a detection signal based on the detected transmitted light. After the adjustment, the culture vessel is irradiated with illumination light for phase-contrast measurement, and the specimen irradiated with the illumination light is imaged.