A laser crystallization apparatus which capable of correcting both shift in imaging position caused by thermal lens effect of the imaging optical system and shift due to flatness of the substrate comprises an crystallization optical system which irradiates laser light to a thin film disposed on the substrate to melt and crystallize an irradiated region of the thin film, the apparatus includes a measurement light source which is disposed outside a light path of the laser light, and which emits measurement light being illuminated the irradiated region of the thin film, and a substrate height correction system which illuminates the thin film with the measurement light through an imaging optical system in the crystallization optical system, and which detects the reflected measurement light from the thin film.