A diagnostic apparatus includes an obtaining unit for obtaining horizontal misalignment information indicative of whether there is horizontal misalignment in a probing beam. The diagnostic apparatus includes a diagnostic unit for diagnosing 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. The vertical direction corresponds to a height direction of the vehicle. The diagnostic apparatus includes a determining unit for determining, based on the horizontal misalignment information, whether the diagnostic unit executes diagnosis of the vertical misalignment. The determining unit causes the diagnostic unit to execute diagnosis of the vertical misalignment upon the horizontal misalignment information representing that there is no horizontal misalignment. The determining unit disables the diagnostic unit from executing diagnosis of the vertical misalignment upon the horizontal misalignment information representing that there is horizontal misalignment.