To provide a power saving device which is capable of preventing the primary winding wire of a transformer from being tap-changed due to a temporal drop in an input voltage so as to restrain lighting from flickering and has an enough power-saving effect on the transformer.

When an drop in an input voltage is detected, the timer of a timer section 6 is made to operate, if an input voltage is returned to a normal state while the timer is kept in operation, the tap changing operation of the primary winding of a transformer 2 is canceled, and if an input voltage is not returned to a normal state while the timer is kept in operation, the tap changing operation of the primary winding of a transformer is carried out after a time previously set by the timer of the timer section 6 elapses, and if an input voltage becomes lower than a previously set lower limit of voltage, the tap changing operation of the primary winding of a transformer is carried out at once without starting the timer of the timer section 6. When a rise in an input voltage is detected, the timer of the timer section 6 starts operating after a certain time elapses in this state, the tap of the primary winding of a transformer 2 is switched and controlled so as to decrease the transformer in output voltage.