A laser processing device (100) comprises a laser light source (101) for emitting a laser light (L) and a laser light source controller (102) for controlling the pulse width of the laser light (L) and irradiates an object to be processed (1) with the laser light (L) while locating a converging point (P) within the object (1), so as to form a modified region along a line to cut (5) of the object (1) and generate a fracture extending in a thickness direction of the object (1) from the modified region as the modified region is formed. In the laser processing device (100), the laser light source controller (102) changes the pulse width of the laser light (L) according to a data table in which the fracture length, the thickness of the object (1), and the pulse width of the laser light (L) are associated with each other. That is, the pulse width is changed according to the fracture length generated from the modified region. Therefore, the laser processing device (100) can generate a fracture having a desirable length from the modified region.