To provide an on-roof lighting device for lighting a shaded yard and an indoor space by effectively using a prism panel for transmitting, refracting and reflecting the light as a lighting device on a flat roof in response to a site and planning condition.

The on-roof lighting device 1 formed of the prism panel 2 formed of a plate-like prism sheet 22, of which one surface is formed with a recessed and projecting surface 22 continuously formed with angle parts 222 and valley parts 223 each other in one direction and of which the other surface is formed of a nearly flat surface 220, and having a characteristic for transmitting and refracting the light is provided on a roof 12 of a building 10a through fixing fittings 5 adjustable in angle and installation direction thereof.