PURPOSE: To obtain an automatic dimmer which makes the automatic dimming of a fluorescent lamp possible in inexpensive constitution.

CONSTITUTION: Light of wavelength about 900nm-1000nm, emitted from a fluorescent lamp 10, is received in an infrared ray remote controlling light receiving device 1 disposed oppositely to the fluorescent lamp 10. A pulse signal in which the number of pulses per unit time varies proportionately to the quantity of the light received, is outputted from the infrared ray remote controlling light receiving device 1. The number of pulses in the pulse signal is counted for a specified time or the pulse widths are added up for a specified time in a microcomputer 4. This computed result and a reference value based on the light quantity to be maintained in the fluorescent lamp 10 are mutually compared. A dimming command for nullifying the difference between the computed result and the reference value in adjustment of light quantity in the fluorescent lamp 10 is outputted to a driver 5 for dimming, so that light quantity in the fluorescent lamp 10 is adjusted automatically.