1. An X-ray CT scan simulator, comprising: a storing device for storing a reference image; a setting device for setting a target noise value and a condition for a simulated lesion specified by an operator; a simulated image generation device for generating a simulation image containing said simulated lesion; and an image display device for displaying the simulation image containing the simulated lesion, wherein said storing device further stores a first tube current and time for the reference image and a projection data noise as a function of a projection data where scanning is performed in a certain tube current and time, and wherein said simulated image generation device further comprises: a generating device for generating a noise image to be added based on a noise value for the reference image and the target noise value; a calculating device for calculating second tube current and time for obtaining the noise image to be added based on the noise value for the reference image and the first tube current and time; a reference projection data generating device for generating reference projection data by re-projecting the reference image; a noise projection data generating device for generating noise projection data based on the function in the second tube current and time and the reference projection data; a simulated lesion projection data generating device for generating a simulated lesion projection data in accordance with the condition for the simulated lesion, which (i) calculates a profile of the simulated lesion in a slice thickness direction, (ii) calculates an integral of the profile, and (iii) generates the simulation image by setting a value of the simulated lesion in accordance with the integral of the profile; an projection data adding device for generating added projection data by adding the noise projection data and the simulated lesion projection data; an image reconstructing device for reconstructing the added projection data and generating an image for addition including the simulated lesion; and a simulation image creating device for creating the simulation image by adding the reference image and the image for addition.