Abstract: A diagnostic apparatus includes a recording device, an acquisition device, and processing circuitry. The acquisition device acquires first and second diagnostic target data obtained in a manner that an image forming device forms a test pattern on each of a first medium and a second smaller medium with the first medium having a long side in agreement with that of the second medium in a conveying direction and an image reading device reads the first medium and reads the second medium rotated such that the long side of the second medium is different in direction from that of the first medium. The circuitry detects an abnormal area and diagnoses an abnormality factor, with the pattern, the first and second data, and third diagnostic target data obtained by rotating the second data such that the long side of the second medium is identical in direction to that of the first medium.