In a voltage source according to the present invention, a switching transistor (Q1) is turned on when the voltage of an electrolytic capacitor (C1) exceeds a predefined positive voltage level (E1), to keep the electrolytic capacitor (C1) at the predefined voltage level (E1), and turned off when the voltage of the electrolytic capacitor (C1) is reduced below the predefined voltage level (E1). A switching transistor (Q2) is turned on when the voltage of an electrolytic capacitor (C2) is reduced below a predefined negative voltage level (-E2), to keep the electrolytic capacitor (C2) at the predefined voltage level (-E2), and turned off when the voltage of the electrolytic capacitor (C2) exceeds the predefined voltage level (-E2). Therefore, an output voltage (V3) is controlled between the predefined voltage level (E1) as an upper limit and the predefined voltage level (-E2) as a lower limit.