An X-ray CT apparatus has a scanner, a pre-scan control unit, an image generating unit, a region setting unit and a timing sensing unit. The pre-scan control unit controls an operation of the scanner to perform a pre-scan. The image generating unit generates image data of the object based upon the pre-scan. The region setting unit sets, on the basis of the image data, a region of interest, and a larger region encompassing the region of interest and a region around the region of interest and being used to determine whether or not a high pixel value region exists. The timing sensing unit senses, if the larger region does not include a pixel value higher than a first threshold, a timing to start a main scan, at which a pixel value of the region of interest exceeds a second threshold after injection of the contrast medium agent is started.