Disclosed is a laser irradiation state diagnosing method which allows accurately diagnosing a laser irradiation state. When irradiating a laser beam so that an irradiation spot scans the surface of the irradiation object, acoustic information in vicinity of the irradiation spot is acquired. And based on characteristics of the acoustic information, such as an intensity of a component of a specific frequency band or a frequency band distribution, a state of peeling of the adhered substances existing on the surface of the irradiation object is determined.