To convert microwave energy into the light beam at high efficiency by installing an input power source for increasing the ratio, between the peak and mean anode currents of a magnetron, to a given value or more, in a magnetron driving power source device.

In a magnetron driving power source, the input power source 10 of a high-pressure transformer 32 is adjusted so that the value of an inductor 8 can be changed to make the mean anode current Imean of a magnetron 4 have about 300mA. The relation between the ratio, (Imax/Imean) between the peak and mean anode currents Imax and Imean, and lamp efficiency F/Pmgt: light beam/magnetron input), is shown in a figure. Consequently, when a power source is made to have an Imax/Imean of 3.75 or more, a lamp efficiency can be made to have 74 (lm/W) or more, to convert micro wave energy into the light beam at high efficiency.