1. An X-ray computed tomography (CT) apparatus having an X-ray tube emitting an X-ray, a detector detecting X-rays transmitted through a subject to be examined, and a bed on which said subject to be examined is placed, said X-ray CT apparatus reconstructing an image of the subject to be examined from a transmission data obtained by detecting the emitted X-rays transmitted through the subject to be examined, comprising: an input part configured to specify an area of the subject which is to be reconstructed in synchronization with a biological signal of the subject and an area which is to be reconstructed in asynchronization with the biological signal; a scan controlling part configured to perform a continuous single helical scan by moving said X-ray tube and said detector relative to said bed, and also to change a moving speed of said bed during the continuous single helical scan; a detecting part configured to detect the biological signal of the subject to be examined; a memory part configured to store the transmission data obtained during said helical scan and the said biological signal data in a correlated way; a segmentation part configured to segment said transmission data into a first transmission data that corresponds to the area to be reconstructed in synchronization with said biological signal and a second transmission data that corresponds to the area to be reconstructed in asynchronization with said biological signal, while a region corresponding to an area on both sides of a boundary between the synchronization area to be reconstructed and the asynchronization area to be reconstructed belongs to both the first transmission data and the second transmission data; a biological-signal synchronization reconstructing part configured to reconstruct a synchronized image of said subject to be examined based on said biological signal data and said first transmission data including data from the region corresponding to the area on both sides of the boundary; a biological-signal asynchronization reconstructing part configured to reconstruct an asynchronized image of said subject to be examined from said second transmission data including data from the region corresponding to the area on both sides of the boundary without using said biological signal data; and a combining part configured to create a combined image from said synchronized image and said asynchronized image such that the synchronized image reconstructed by said biological-signal synchronization reconstructing part and the asynchronized image reconstructed by said biological-signal asynchronization reconstructing part are combined with weighting addition at said region corresponding to the area on both sides of the boundary.