A light irradiation device including an injection unit to inject a sample, and a light source to apply light to the sample to identify an abnormal cell in the sample, the light source including a substrate and a light emitter including a light emitting diode, in which the light emitted is to cut a genetic material in the sample into sections of different sizes, and to deform the genetic material to different degrees, such that a determination of an abnormality of the sample is based on the degree of deformation, and an irradiation amount or intensity of the light is at an intensity in which a cytotoxicity value of the sample is greater than or equal to a predetermined value, and a ratio of a degree of deformation of the genetic material of the normal sample to that of the abnormal sample is set to be a minimum value.