A target object detecting device (1) is provided, which may include an acquisition part (25), a generation part (31a), and a detecting part (31b). The acquisition part (25) may acquire echo signals from target objects around a ship. The generation part (31a) may generate a first echo image based on the echo signals. The detecting part (31b) may input the first echo image into a model (41) built by machine learning, and may detect a first target object that is a target object other than a ship corresponding to the model (41), based on an output from the model.