A method of laser beam machining a machining target comprising: a step of irradiating laser light to the machining target while converging the light to the inside of the machining target; wherein the machining target is a semiconductor substrate, wherein the laser light is transmitted through a surface of the machining target and a peak power density of the focusing point of the laser light is equal to 1 x 10 8 W/cm 2 or more, a step of cutting the machining target which is formed with the minute cavity, characterised in that the laser light is a pulse laser light and the laser light is relatively moved along the planned cutting line at a predetermined pitch to thereby move a focus point along the planned cutting line, whereby the treated area and the minute cavity area are formed inside the machining target, wherein the pulse width of the laser light is set to 1 μsec or less and the pulse pitch of the laser light is set to 3,00 μm to 5,00μm, so as for forming a treated region which includes a treated area in the machining target along a planned cutting line of the machining target and forming a minute cavity region which includes the minute cavity along at least one part of the planned cutting line, wherein the treated area is a molten processed area, wherein the minute cavity is formed on the other side of the laser light incidence, putting the treated area between them and wherein each of the minute cavities are separated by a predetermined interval along the planned cutting line, the machining target is cut by forming the treated region and the minute cavity region as a starting point of cutting, and wherein an expand tape is attached to the backside of the machining target and is expanded in a peripheral edge direction of the machining target to separate the machining target into parts along the planned cutting line, thereby cutting the machining target, so that after cutting the machining target, there are minute cavities in the cut surfaces. a step of irradiating laser light to the machining target while converging the light to the inside of the machining target; wherein the machining target is a semiconductor substrate, wherein the laser light is transmitted through a surface of the machining target and a peak power density of the focusing point of the laser light is equal to 1 x 10 8 W/cm 2 or more, a step of cutting the machining target which is formed with the minute cavity, characterised in that the laser light is a pulse laser light and the laser light is relatively moved along the planned cutting line at a predetermined pitch to thereby move a focus point along the planned cutting line, whereby the treated area and the minute cavity area are formed inside the machining target, wherein the pulse width of the laser light is set to 1 μsec or less and the pulse pitch of the laser light is set to 3,00 μm to 5,00μm, so as for forming a treated region which includes a treated area in the machining target along a planned cutting line of the machining target and forming a minute cavity region which includes the minute cavity along at least one part of the planned cutting line, wherein the treated area is a molten processed area, wherein the minute cavity is formed on the other side of the laser light incidence, putting the treated area between them and wherein each of the minute cavities are separated by a predetermined interval along the planned cutting line, the machining target is cut by forming the treated region and the minute cavity region as a starting point of cutting, and wherein an expand tape is attached to the backside of the machining target and is expanded in a peripheral edge direction of the machining target to separate the machining target into parts along the planned cutting line, thereby cutting the machining target, so that after cutting the machining target, there are minute cavities in the cut surfaces.