According to an embodiment, a diagnostic apparatus includes a sound-emitting unit, at least one measurement unit, and a processor. The sound-emitting unit includes a plurality of speakers arranged at equal angular intervals on a circumference of a circle, and is configured to emit a first vibration sound to a target by using the speakers. The at least one measurement unit is arranged on a central axis of the circle, and is configured to measure a vibration of the target generated in response to the first vibration sound, or a second vibration sound radiated from the target due to the vibration. The processor is configured to diagnose the target based on an output from the at least one measurement unit.