An image processing apparatus includes an image processing section periodically performing image processing for a periodically captured image, a diagnosing section comparing an image processing result obtained from diagnostic image data with an expected value data which indicates a reference for a normal processing result of image processing of the diagnostic image data and determining whether the image processing result obtained from the diagnostic image data is normal, by making the image processing section perform image processing for the diagnostic image data which is directly accessible by the image processing section in parallel with image processing periodically performed by the image processing section for the captured image, and an output controlling section outputting the processing result of image processing for the captured image as valid to a control section on condition that the image processing result obtained from the diagnostic image data is determined as normal by the diagnosing section.