1. An X-ray CT apparatus, comprising: an X-ray tube configured to emit X-rays; a high-voltage power supply configured to apply a tube voltage to the X-ray tube; an X-ray detector including a plurality of X-ray detection elements, and configured to detect the X-rays; and a processing circuitry configured to: reconstruct, as a first image, any of a monochromatic X-ray image and a polychromatic X-ray image on a basis of pre-reconstruction data acquired by scanning an object, the polychromatic X-ray image being taken in a first X-ray energy band; reconstruct, as a second image, a polychromatic X-ray image taken in a second X-ray energy band wider than the first X-ray energy band on a basis of a pre-reconstruction data acquired by scanning the object; identify an area of a correction target in the first image as a correction area; and correct the correction area in the second image on a basis of the first image.