1. An X-ray CT apparatus having an X-ray source and an X-ray detector disposed to face said X-ray source with a subject placed therebetween, for rotating at least one of said X-ray source and said X-ray detector around said subject and detecting cone-shaped X-rays emitted from said X-ray source toward said subject by said X-ray detector, comprising: a defining device for defining a reconstruction plane for said subject in a plane non-parallel to a plane formed by rotation of at least one of said X-ray source and said X-ray detector around said subject.