The present invention relates to an illuminating device that comprises a ballast (1), a driving means (2) connected with the ballast, and an illuminating load (3) driven by the driving means, wherein the ballast comprises a first coil unit (11) and a second coil unit (12a, 12b) coupled with the first coil unit, wherein the first coil unit outputs a first current from a power source to the driving means and the second coil unit, and the second coil unit outputs a second current to the driving means according to the first current, wherein the driving means comprises an auxiliary module (21a, 21b) receiving the first current and the second current, wherein the auxiliary module is configured to reduce the resonant frequency of the second coil unit so as to reduce the second current.