PURPOSE: To provide a lamp turn-off detection device capable of remarkably enhancing the work efficiency of workers in charge of maintenance by checking nonconformity of lamps such as streetlights in the daytime, and enabling replacement of the lamps on the spot.

CONSTITUTION: As it gets dark, a photosensor portion 13 detects it and turns on a switch 2 to light a lamp 4. Then the relay 62 of an abnormality detecting portion 6 is turned on to actuate the abnormality detecting portion 6. Thereafter, if the current detector 61 of the abnormality detecting portion 6 does not detect normal current, a transistor 63 is turned on and the thyristor 12 of a control portion 30 is turned on to make the light-emitting element 11 of a display portion 40 emit light, thereby indicating abnormality of the mercury lamp 4. Even if the switch 2 is turned off as it gets brighter, lighting of the light- emitting element 11 is maintained by power supply from the battery 9 of a backup power source portion 20, whereby abnormality of the mercury lamp 4 is indicated even in the daytime.