A dimming circuit for a gas-discharge lamp includes a first terminal and a second terminal for inputting AC power, a variable resistor and a capacitor connected with the respective first end to each other and the respective second end to the first input terminal and the second input terminal respectively, a bi-directional diode connected with one end to the first end of the variable resistor and the first end of the capacitor, a TRIAC, which has the gate thereof connected to the second end of the bi-directional diode and one end connected to the second input terminal, and a switch connected in series between the second input terminal and the capacitor for circuit on/off control, for enabling the light intensity of the gas-discharge lamp to be regulated subject to change of the resistance value of the variable resistor.