In a container of a cell culture device, a light source and a camera are arranged on both sides of an incubator, and the camera or incubator can be moved in relation to each other.An image of cells in the incubator is captured with a given focus.The medium, image density, and noise component of images captured while varying the focus of the camera have substantially similar pixel values.Consequently, by subjecting the data on the images captured with such different focuses to differential processing, only the cell portion can be extracted without being influenced by the color variation of the medium, the variation of the amount of light, the difference in brightness between the central and peripheral areas of the image, and the noise.