In this paper, we analyzed the vulnerability of information leakage due to the leakage electromagnetic waves of a PC keyboard. First, we reviewed the keyboard protocol and hardware structure, we analyzed the correlation between the data signal, which is transmitted from the keyboard to the main body, and the leakage signal on the power cable. With the result, we grasped the cause of the Conducted Emission of a PC keyboard. Also, we compared the limit level of the CISPR 22 standard with the amplitude of the keyboard leakage electromagnetic waves we calculated. By analyzing the signal on the power cable of the PC main body through the simple experiment, we show that it is possible to extract the contents of the PC key. Therefore it is verified that the secret information of the PC user could leak out.