Processing along one circular path is performed by irradiating a laser beam to a ceramic green sheet attached on an outer peripheral surface of a rotating drum, and the processing path is changed by moving at least one of the rotating drum and a beam irradiated position through a predetermined distance in the direction of drum centerline each time the processing on one circular path is completed. If the laser beam is intermittently irradiated to the ceramic green sheet, a matrix of through holes or concave portions can be easily formed in the ceramic green sheet.