In order to confirm whether electromagnetic (EM) waves are excited by earthquakes or not, we have been observing EM signal in the earth, using an EM sensor system installed into a deep borehole. By shifting down the monitoring frequency from VLF to ELF range in December 2011, we have been able to detect EM signals excited by seismic waves, and have clarified the excitation mechanism of the EM pulses in the earth. Through detailed analysis of the EM signals based on the clarified behaviors of EM waves excited by seismic waves, we have found a method of deriving electrical parameters of the earth's medium.