A machining head emits a laser beam for cutting sheet metal of stainless steel. A moving mechanism moves the machining head relatively to a surface of the sheet metal. A beam vibrating mechanism vibrates a laser beam in a parallel direction with a cutting advancing direction of the sheet metal. In a machining condition database, a single specific vibration frequency at which cutting of the sheet metal is possible is set to a maximum moving velocity at which cutting of the sheet metal is possible, and a plurality of vibration frequencies from a maximum frequency to a minimum frequency at which cutting of the sheet metal is possible are set to a moving velocity more than or equal to a minimum moving velocity and less than the maximum moving velocity at which cutting of the sheet metal is possible.