Toward a battery-free wearable system, we have investigated a semi-passive human body communication wherein power and information are transmitted via the human body. In particular, we evaluated the effect of the position of the wearable device, the contact between the body and electrodes, and the contribution of the floor ground (GND) on the transmission between the HBC transceivers, using both subject experiment and numerical simulation. The results confirmed that, depending on the usage conditions, the floor GND deteriorates the transmission characteristics by increasing the electrical field that does not contribute to communication.