A method for forming a median crack and an apparatus for forming a median crack are provided, where the formation of a deep, straight median crack is possible, and an excellent broken surface of a brittle substrate can be gained as a result of breaking.In the method for forming a median crack in a brittle substrate, the brittle substrate is irradiated with a laser beam along a laser-scribe line to be formed with median cracks on the brittle substrate so as to be heated to a temperature that is no higher than its melting temperature, and thereby, a median crack is created along the laser-scribe line to be formed with median cracks and expanded starting from a notch that has been formed in the brittle substrate, and this method for forming a median crack in a brittle substrate is characterized in that high temperature portions that receive intense radiation from a laser beam, and low temperature portions that receive faint radiation from a laser beam are alternately formed along the laser-scribe line to be formed with median cracks.