An X-ray CT apparatus reconstructs a CT image (extended CT image) at each slice position lying outside a relative linear movement range at a helical scan or the like at low exposure and with high image quality. Scans each provided with a residence time are executed at relative linear movement start and end points (Zs and Zf). Projection data in a view range (F) corresponding to a predetermined view angle centering on a view (CVp1,zs) corresponding to a relative linear movement start time or a view shifted previously therefrom by the number of views (M), or a view (CVp1,zf) corresponding to a relative linear movement end time or a view shifted backward therefrom by the number of views (N) are used as image reconstruction projection data for extended CT images in image extension regions (Rs and Rf).