To protect a warmer from being damaged by so-called empty burning by alarming shortage of liquid fuel.

Reflecting faces 5, 6 of mirror finish for directing the reflected light ward the counterpart element are disposed in the main axial direction of the function of light emitting element 3 and photodetector 4. An inclining face for totally reflecting the light from the light emitting element 3 and preventing the light from entering the photodetector 4 when touching the air is interposed between the light emitting element 3 and photodetector 4. Erroneous operation is prevented by inverting the output such that the light from the light emitting element 3 reaches the photodetector 4 through the reflecting faces 5, 6 and inclining face when the sensor of touching a light and inclining face whereas the light from the light emitting element is totally reflected on the inclining face and does not reach the photodetector 4.