PROBLEM TO BE SOLVED: To enhance the accuracy of collision avoidance.SOLUTION: A figure shows the propagation characteristic of a radio wave in a collision avoidance system in which an avoidance device is installed on a power-assisted car and a detection object device is held by a pedestrian. Because a magnetic field passes through a wall without reflection, the avoidance device can communicate with a detection object device (#1) behind a wall or pillar. The strength of the magnetic field has an attenuation characteristic of 60 dB/dec when a typical loop antenna is used. Therefore, it is hardly influenced by the variation of reception circuits, and the boundary of the communication range of the magnetic field becomes clear. Thus, a detection object device (#2) out of the communication range can be prevented from receiving the magnetic field. In other words, a scheme using a magnetic field can clarify a detection area and is influenced little by the shielding and reflection by an obstacle, and thereby can accu
