In an apparatus installed on a boat (A) in plural boats including (A, B) for avoiding collision with the boat (B), there are equipped with first and second radio-wave transceiver units installed at spaced positions that transmit to and receive radio waves of a predetermined frequency from the boat (B), and first and distance calculating units that calculate clearance distances from the boat (B) based on attenuation degree of the radio waves transmitted from the boat (B). An intersection point of circles whose origins in an x-y coordinate plane are set to positions corresponding to the first and second positions and whose radii are set to values corresponding to the calculated clearance distances. Direction and position of the boat (B) are estimated from the intersection point, and an instruction necessary for avoiding collision with the boat (B) is issued based on the estimated direction and position.