Error that occurs when an absolute velocity of a target object is measured by using an antenna installed on a ship body that rocks and drifts in a complex manner since it floats on the sea is reduced. An antenna is installed on a ship body and transceives electromagnetic waves. A roll angle and a pitch angle of the ship body are detected by using an inclination sensor. An antenna velocity calculator calculates an antenna velocity of the antenna by using the detected roll and pitch angles of the ship body. An antenna velocity compensator compensates the antenna velocity of the antenna for a relative velocity between the ship body and a target object, the antenna velocity calculated by the antenna velocity calculator, the relative velocity obtained based on reflection waves received by the antenna.
