To conduct efficient laser beam machining by reducing a laser beam to be shut off with a mask in laser beam machining.

A laser beam machining is so that by passing a laser beam through a mask arranged with an opening, is reshaped to a prescribed cross sectional shape, the machining object face is irradiated with the laser beam. Plural cylindrical lenses 105, 106 are arranged from a laser beam oscillator to the mask so that the axes are vertical to the optical pass and its axis directions are different each other, by passing these plural lenses, the cross sectional shape is changed so as to reduce the portion shut off with the masks, the laser beam is guided to the mask.