A method of in-situ monitoring of a crystallization state is used for laser anneal processing for applying an energy line irradiation for at least one of crystallization of a thin film and promotion of the crystallization. The method of in-situ monitoring of a crystallization state is characterized by irradiating simultaneously at least a plurality of monitoring places in a region having a predetermined area of at least one of the surface and the underside of the thin film by a monitor light for monitoring a crystallization state of the thin film at least during or after of before, during and after the energy line irradiation directly or through a substrate, and measuring a temporal change of the intensity of at least one of a reflected light and a transmitted light, from the surface or the underside of the thin film, of the monitor light as a light intensity distribution related to the positions of the monitoring places.