1. A cell monitoring device, comprising: an outline detecting section that detects edge pixels from a cell image in a captured image of cells arranged in a single layer and generates an edge image including the detected edge pixels; a pigmented region detecting section that detects pixels of a pigmented region of the cell image in the captured image, and generates a pigmented region image including the detected pixels of the pigmented region; and an image segmentation section which overlays the edge image and the pigmented region image, removes extra edge portions in the edge image, performs completion of incomplete cell shapes in the edge image, whereby generates a new edge image, and performs a segmentation of the captured image based on the new edge image; and a detection section which determines an intensity variance of pixels in each of segments in the captured image following a result of the segmentation, and determines cell segments and background segments in the captured image.