A radar comprising: an antenna for emitting beams of electromagnetic waves into space in which a target is present, and receiving electromagnetic waves that are reflected from the target and return to the antenna; a signal processing unit for performing signal processing to detect the target from the electromagnetic waves received by the antenna; an information acquiring unit for acquiring, from a further sensor adapted to detect the target, direction information indicating a direction in which the target is to be searched for and observation accuracy information indicating observation accuracy in the direction; a beam arrangement determining unit adapted to determine an arrangement of beams to be emitted by the antenna from the direction information and the observation accuracy information acquired by the information acquiring unit; and a beam controlling unit for controlling directions of the beams to be emitted by the antenna in accordance with the arrangement of beams determined by the beam arrangement determining unit, wherein the beam arrangement determining unit is adapted to: calculate a presence probability of the target from the direction information and the observation accuracy information acquired by the information acquiring unit, calculate a detection probability of the target from an antenna pattern of the antenna for each of beam arrangement candidates each representing an arrangement candidate for the beams, average the detection probability of the target in each of the beam arrangement candidates with the presence probability of the target, select a beam arrangement candidate having a relatively high averaged detection probability of the target from among the beam arrangement candidates, and determine the selected beam arrangement candidate to be an arrangement of the beams to be emitted by the antenna. an antenna for emitting beams of electromagnetic waves into space in which a target is present, and receiving electromagnetic waves that are reflected from the target and return to the antenna; a signal processing unit for performing signal processing to detect the target from the electromagnetic waves received by the antenna; an information acquiring unit for acquiring, from a further sensor adapted to detect the target, direction information indicating a direction in which the target is to be searched for and observation accuracy information indicating observation accuracy in the direction; a beam arrangement determining unit adapted to determine an arrangement of beams to be emitted by the antenna from the direction information and the observation accuracy information acquired by the information acquiring unit; and a beam controlling unit for controlling directions of the beams to be emitted by the antenna in accordance with the arrangement of beams determined by the beam arrangement determining unit, wherein the beam arrangement determining unit is adapted to: calculate a presence probability of the target from the direction information and the observation accuracy information acquired by the information acquiring unit, calculate a detection probability of the target from an antenna pattern of the antenna for each of beam arrangement candidates each representing an arrangement candidate for the beams, average the detection probability of the target in each of the beam arrangement candidates with the presence probability of the target, select a beam arrangement candidate having a relatively high averaged detection probability of the target from among the beam arrangement candidates, and determine the selected beam arrangement candidate to be an arrangement of the beams to be emitted by the antenna. calculate a presence probability of the target from the direction information and the observation accuracy information acquired by the information acquiring unit, calculate a detection probability of the target from an antenna pattern of the antenna for each of beam arrangement candidates each representing an arrangement candidate for the beams, average the detection probability of the target in each of the beam arrangement candidates with the presence probability of the target, select a beam arrangement candidate having a relatively high averaged detection probability of the target from among the beam arrangement candidates, and determine the selected beam arrangement candidate to be an arrangement of the beams to be emitted by the antenna.