An indoor lighting device of a vehicle includes: a substrate, a plurality of light sources assembled with the substrate to be arranged in a row, and a light guide panel including: a plurality of incidence parts such that light emitted by the light sources is incident thereupon, a diffusion area part provided behind the incidence parts to diffuse and reflect the light incident upon the incidence parts, a visual field area part provided behind the diffusion area part to emit the light diffused and reflected by the diffusion area part outside, and light diffusion holes provided behind the respective incidence parts. In particular, each of the light diffusion holes is surrounded by a transmission plane, a pair of reflection planes and a diffusion plane.