To improve lighting of a fluorescent tube in its start-up, and certainly light the fluorescent tube even in the case where a discharge time lag arises.

In a piezoelectric inverter circuit, a difference defined between a tube current which flows a fluorescent tube 30 and a reference value processed by a processing means 50 for the reference value is calculated by a tube current sensing unit 40, and accordingly a control means 10 for the frequency controls a frequency of a voltage applied to a piezoelectric transformer 20. The processing means 50 for the reference value, for a certain time for the start-up, gives such a start-up reference value obtained by adding a prescribed value on a normal reference value. Thereby, a control means 10 for a frequency generates a release voltage for certainly lighting the fluorescent tube 30 to the piezoelectric transformer 20 for a constant time.