An image processing device determines the degree of activity of a cell during cell viewing. An imaging device captures first and second images of a cell under observation. After aligning the rotational angle orientations on the image plane of the cell under observation that is included in the images, a correlation value or difference of intracellular texture feature values in the first image and second image, is sequentially computed. A time series variation of the computed correlation value or difference is then derived, and the degree of activity of the cell under observation is detected based on the state of variation.