The present invention: assesses whether a test cell is a normal cell or a cancerous cell by irradiating laser light on the test cell and using the difference in the shape formed by the various points, which plot the laser light phase differences on a straight line measured for all pixels of the cells in a microscope viewing field on an X-Y coordinate plane, as an index; or assesses whether the test cell is a normal cell or a cancerous cells by setting respective assessment criteria on an X-Y coordinate plane for normal cells and cancerous cells in advance and using the difference between the phase difference value measured with the test cell on the X-Y coordinate plane and the assessment criteria values as an index.
