We focus on the hardness measurement by using a sensor unit arranging 8 piezoelectric elements in series. In previous study, it confirmed that the sensor unit is available to evaluate roughness on the sample. In the study, four silicone samples with different Young's modulus were prepared. In experiments, the sensor unit was placed on the sample and moved up and down with 1Hz. And the oscillatory waveform form each piezoelectric element was obtained. In analysis, the first peak voltage in the waveform were extracted and compared with the Young's modulus of the sample. From the results, it was found that the peak voltage related on Young's modulus. It was suggested that the sensor unit is available to evaluate the hardness of objects.