PURPOSE: To obtain a highly efficient signal-waveform measuring apparatus by performing differential amplification under the optimum state with respect to the signal-waveform measuring apparatus which can observe a high-speed electric signal by utilizing an electrooptical effect, especially with regard to the signal-waveform measuring apparatus wherein measuring accuracy and operability are improved.

CONSTITUTION: A signal-waveform measuring apparatus has a laser light source 1, photodetectors 6 and 7 and a differential amplifier circuit 10. The change in electric field induced in an electropotical crystal 4, which is arranged close to or in contact with an object 20 to be measured, is made to be the change in polarized state of light which travels back and forth in the crystal 4. The amount of the difference between the intensities of a pair of signal lights. which are obtained by polarizing and splitting the reflected light, is detected. The waveform of the voltage, which is applied on the object 20 to be measured is measured. Lower limiting circuits 8 and 9, which do not sense the specified limit value or less and pass the signal higher than the limit value, are provided. A control circuit 13 for setting the limit values of the lower limiting circuits 8 and 9 is provided.