The switching power source circuit (11) for the solid-state laser (100) excited by a semiconductor laser (101) uses a higher switching frequency than the relaxation oscillation (fRO) of the solid-state laser so that the optical noises due to the switching noises or ripples in the output voltage of the switching power source circuit can be minimized. When the voltage drop caused by a semiconductor power device (Q2) is forwarded to a feedback terminal (11b) of the switching power source circuit, even when the forward voltage of the semiconductor laser should vary, the output voltage of the switching power source circuit can be regulated to a value suitable for the driving of the semiconductor laser, and the heat generation from the semiconductor power device can be minimized.