Cell observation apparatus and imaging method used in cell observation apparatus

An imaging method used in a cell observation apparatus including a placement unit, an imaging optical system, a first movement unit, and an imaging unit, the imaging method comprising:

• forming, for a cell culture vessel containing a cell that is placed on the placement unit as an object to be observed, an image of the object to be observed using the imaging optical system that forms an observation image of the object to be observed;
• moving at least one of the placement unit and the imaging optical system along an optical axis direction of the imaging optical system from a movement start position to a movement end position;
• taking a plurality of images of the observation image in movement in the optical axis direction using an image sensor of the imaging unit;
• evaluating a plurality of taken images based only on a distribution degree of luminance values within a predetermined luminance value range;
• and
• selecting a taken image with a highest distribution degree of luminance values as an in-focus image.