1. A CT apparatus for making a plurality of helical scans about an object under examination placed on a support table, and collecting, through each scan, CT image composing data covering a scanning range of 180° for each point in an area of interest, said CT apparatus comprising: electromagnetic wave emitting means for emitting electromagnetic waves in a conical form toward said object; planar detecting means opposed to said electromagnetic wave emitting means across said object for detecting the electromagnetic waves emitted from said electromagnetic wave emitting means and diverging two-dimensionally; drive means for moving said electromagnetic wave emitting means and said planar detecting means relative to said object while rotating said electromagnetic wave emitting means and said planar detecting means about said object, to cause electromagnetic waves to make helical scans around said object, whereby CT image composing data covering a 180° scanning range for each point in the area of interest of said object are collected from said planar detecting means; plural scan control means for controlling said drive means such that the plurality of helical scans made of said object by said electromagnetic wave emitting means have helical paths with a phase difference therebetween of 360° divided equally; and image reconstructing means for performing an image reconstructing process based on CT image composing data collected through said plurality of helical scans.