When first grooves are formed by irradiating a lower surface facing a light outgoing surface of a light guide plate for an illuminating device with laser beams, an outer circumferential region is firstly irradiated with the laser beams to form second grooves. Then, the second grooves are inspected. Next, irradiation conditions of the laser beams are adjusted based on an inspection result of the second grooves, irradiation positions of the laser beams are linearly displaced from formation positions of the second grooves, and formation of the first grooves in the scattering reflection region is then started.