Recently, wearable devices which use the human body as a transmission channel have been developed. However, there has been a lack of information related to the transmission mechanism of such devices in the physical layer. Electromagnetic communication trials using the human body as a transmission medium have more than a decade's history. However, most of the researches have been conducted by researchers who just want to utilize the fact and practically no physical mechanisms have been researched until recently. Hence, in this paper, the authors propose some calculation models of the human body equipped with the wearable devices by using the finite difference time domain (FDTD) method. Moreover, a biological tissue-equivalent solid phantom is utilized to show the validity of the calculation. From these investigations, the authors determine the transmission mechanism of the wearable devices using the human body as a transmission channel.