1. An X-ray CT scan simulator, characterized by comprising: an image storing device for storing a reference image; a target noise value setting device for setting a target noise value of a desired image; a simulated lesion setting device for setting a condition for a simulated lesion postulated by an operator; a simulation image generating device for generating a simulation image containing a simulated lesion by using the reference image on the basis of the set target noise value and the condition for the simulated lesion; and an image display device for displaying the simulation image containing the simulated lesion.