PROBLEM TO BE SOLVED: To immediately make the reversibility of a biological tissue determinable even without performing continuous measurement.SOLUTION: A diagnostic device for irradiating a laser beam to a biological tissue, receiving light diffusely reflected from the biological tissue and determining the reversibility of the biological tissue on the basis of an intensity distribution of the light includes an area setting part for setting a determination area in the biological tissue, a laser beam irradiation part for irradiating a laser beam to the determination area, a scattered light reception part for receiving scattered light of the laser beam diffusely reflected from the biological tissue, a scattered light intensity distribution detection part for detecting an intensity distribution in the determination area of the scattered light received by the scattered light reception part, a distribution degree calculation part for calculating the expansion degree of the intensity distribution from the intensity distribution within the determination area detected by the scattered light intensity distribution detection part, and a reversibility determination part for determining that the reversibility of the biological tissue within the determination area is lost or lowered in the case that the expansion degree is larger than a predetermined size.
