To provide a guide light system in a tunnel of a road capable of positively lighting or emitting light in a disaster or the like wherein a lifeline is cut in a tunnel of a road.

The guide light system 1 in a tunnel of a road is provided with a light source lamp 2 comprising a flash lamp, a light emitting circuit 3 lighting the light source lamp 2, a change over circuit 4 turning feeding to the light source lamp 2 on and off, and an illuminance detecting means 7 comprising a light receiving sensor 5 and a detection circuit 6. When illuminance detected by the illuminance detecting means 7 becomes a predetermined level or less, the change over circuit 4 which is normally held in an off state is changed over to on, and the light source lamp 2 is lighted by feeding from a battery power source comprising a primary battery 8. The guide light system 1 in a tunnel of a road can be installed in a target place irrespective of existence of laying of a commercial AC power source, and it can safely guide people since the light source lamp 2 can be positively lighted in a disaster (an emergency wherein a guide light is needed) or the like wherein the commercial AC power source is cut off.