A target object detection apparatus (20) includes an acquisition unit (210), a likelihood information generation unit (220), and a determination unit (230). The acquisition unit (210) acquires an IF signal from an irradiation apparatus (10) at a plurality of timings. The likelihood information generation unit (220) executes, by processing the IF signal, processing of generating, with respect to each of a plurality of IF signals, likelihood information for a region (hereinafter, referred to as a target object region) having a possibility in that an accompaniment, i.e., a target object is present. The likelihood information indicates a distribution of presence probabilities of a target object in at least a height direction in a target object region. Then, the determination unit (230) determines a presence or absence of a target object, by using likelihood information generated for each of a plurality of target object regions.