1. A target detection apparatus, comprising: a first target detection section which detects an object which exists ahead of a vehicle in a first detection area; a second target detection section which detects a target in a second detection area, the second detection area being different from and overlapping with the first detection area, the second detection area having an overlapping detection area that overlaps with the first detection area; and a reliability degree setting section which sets a target reliability degree indicating a probability that the target exists and has a height sufficient to strike against the vehicle, wherein the reliability degree setting section updates the target reliability degree from an initial target reliability degree every time the first target detection section detects the object, when the object is detected by the first target detection section and the target is not detected by the second target detection section, if the object is in the overlapping detection area, the reliability degree setting section lowers the target reliability degree, when the object is detected by the first target detection section and the target is not detected by the second target detection section, and when the object is not in the overlapping detection area, if a stable recognition count value, which is incremented when the object is detected by the first target detection section and when the target is detected by the second target detection section and which is not changed or is decremented when the object is detected by the first target detection section and the target is not detected by the second target detection section, is less than at least one threshold value, the reliability degree setting section lowers the target reliability degree by a predetermined value, and when the object is detected by the first target detection section and the target is not detected by the second target detection section, and when the object is not in the overlapping detection area, if the stable recognition count value is equal to or more than the threshold value, the reliability degree setting section keeps the target reliability degree, or lowers the target reliability degree by a range smaller than a range set when the stable recognition count value is less than the threshold value.