A systematic standard for developing light extraction technologies that can improve the efficiency of lighting systems based on the optical design method of flat-type lighting devices needs to be established. The design of a flat-type lighting device was investigated, considering the utilization of precise processing techniques that simplify the manufacturing process. These techniques adjust the processing variables of the geometric pattern of the light guide plate (LGP), light-emitting diodes (LEDs), which are placed on the sides of the LGP, and viewing angle. The amount of light lost was reduced by etching the light-incident surface of the LGP as a curved shape. A pattern spacing of 9 mm was used to control four LEDs placed in the middle of each side of the LGP with a viewing angle of 120 deg and four LEDs placed at the corners with a viewing angle of 60 deg. In addition, hotspot generation was prevented by controlling the amount of light concentrated in the light-incident area, and the illuminance uniformity of the LGP was confirmed to be 95%, showing an improved optical efficiency. By controlling only the pattern spacing, position of the light source, and viewing angle of the LEDs, the optimal design conditions to improve the optical efficiency of the flat-type lighting device were established. It was found that these parameters can sufficiently improve the imbalance caused by the concentration and loss of light.