To provide a dimming switch by which flicker of luminance of illumination load due to voltage fluctuation of an a.c. power source is prevented.

A series circuit constituted of a Triac TRC and an incandescent lamp L a diode bridge DB are connected in parallel between an electric power source terminal t1 and a common terminal t3. A series circuit constituted of R2, R1 is connected between d.c. output terminals of the diode bridge DB. A control circuit part 4 reads in a set voltage V1 of a variable resistor VR fed to an input terminal IN1 and selects the output voltage from a table set in the inside corresponding to the voltage V1. The control circuit part 4 reads in the voltage V2 of the connection points of the resistor R2, R1 from an input terminal IN2, detects the voltage fluctuation of the a.c. power source AC from the voltage V2, and controls the phase angle of the electric conduction of the Triac TRC so as the output voltage supplied to the incandescent lamp L to be a desired output voltage.