For the purpose of displaying a more accurate exposure dose value without imposing additional work on an imaging operator, an exposure dose calculating method in an X-ray CT apparatus comprises the steps of: inputting the age and a region to be examined of a subject; calculating the head or chest diameter of the subject based on the age and region to be examined of the subject input at the inputting step, and on statistics on the head or chest circumference of a human body; calculating an exposure dose of a phantom having a diameter equivalent to the calculated diameter of the head or chest, based on prior information on an exposure dose to a phantom having a predetermined diameter when a predetermined X-ray dose is applied to the phantom; and displaying the exposure dose value.