A signal level adjusting apparatus is provided. The signal level adjusting apparatus uses a clamping circuit as an automatic level adjusting apparatus to adjust a signal level. Then, a peak or a valley voltage of a driving signal output from the signal level adjusting apparatus can be confined in a predetermined voltage value range. The present invention adjusts an input driving signal automatically according to the predetermined voltage value so as to output an organic light emitting diode (OLED) driving signal suitable for driving various OLEDs and various OLED displays or output a light emitting diode (LED) driving signal suitable for driving various LEDs. Additionally, the present invention further comprises a power amplifier circuit, which is suitable for enhancing a driving ability of the level adjusting apparatus.