In a diagnostic apparatus, a diagnostic unit diagnoses whether there is vertical misalignment. The vertical misalignment is misalignment of the probing beam with respect to a designed beam axis position in a vertical direction, i.e. a height direction, of the vehicle. Based on detection performance information representing whether target detection performance by the beam sensor is likely to be lower than a predetermined detection performance, a determining unit causes the diagnostic unit to execute diagnosis of the vertical misalignment upon the detection performance information representing, as a first detection state, that the detection performance is not likely to be lower than the predetermined detection performance. The determining unit disables the diagnostic unit from executing diagnosis of the vertical misalignment upon the detection performance information representing, as a second detection state, that the detection performance is likely to be lower than the predetermined detection performance.