To do precise dimming in an inexpensive constitution be feeding a signal from a monitoring operation side to selectively operate plural relays set up at a luminaire side.

At a monitor side B, plural current-carrying control switches 7 are provided on a monitor board 3 to be open/closed for every dimming unit with dimming operation. At a luminaire side A including a ceiling, coils 10 for relays 9 corresponding to the current-carrying control switches 7 for every dimming operation unit is connected to the contacts of the corresponding current-carrying control switches 7 via contact signal lines 8. The plural current- carrying control switches 7 which can set lighting steps equivalent to the number of their opening/closing combinations specifies four lighting control steps with the combination of two current-carrying control switches 7, e.g. A lighting control signal generator 12 is made up with a relay 9 contact group and a fixed resistor group to selectively connect the fixed resistor group corresponding to the specified dimming steps for feeding a resultant resistance value as a dimming step signal to a lighting controller 2.