This paper describes a wireless behavioral monitoring system equipped with an omnidirectional electric field sensor. The system can be installed by individuals in their residences. It is composed of a sensor circuit, wireless data transmitter, and receiver that is connected to a personal computer for data storage. The sensor circuit has been designed to obtain information on the usage of domestic appliances by measuring the electric field around them. It is assumed that the usage statistics provide information on a type of indoor behavior of the subject. Since the system employs a device that transmits the obtained data through weak radio waves, the sensor unit, which is composed of the sensor circuit and wireless data transmitter, can be used by simply attaching it to an appliance. Simple evaluation tests confirm the practicability of the system.