The laser machining device (1) is a device for forming a modified region (R) in a machining object (S) by irradiating laser light (L) on the machining object (S). The laser machining device (1) is provided with: a laser light source (2) for emitting the laser light (L); a loading support (8) for supporting the machining object (S); and an optical system (11) for concentrating, of the laser light (L) emitted from the laser light source (2), a circular section, which surrounds the central section comprising the optical axis of said laser light (L), on a specified section of the machining object (S) supported on the loading support (8). The optical system (11) makes it possible to precisely and efficiently form fine modified regions by adjusting the shape of the inner edge and/or the outer edge of the circular region of the laser light (L) according to the position of the specified section in the machining object (S).