This system is a computer-based system for simulating an effect of a specific surgical treatment based on three-dimensional data of a shape of a target vascular site of a subject, comprising: a surgical treatment receiving unit, by a computer, for displaying the three-dimensional data of a shape of a target vascular site on a computer display screen, and receiving a specification of a lesion on display and a selection of a surgical treatment for the lesion, a modification method storage unit, by a computer, for pre-storing selectable surgical treatments and methods for modifying the three-dimensional data for respective surgical treatments in memory, and a modified three-dimensional data output unit, by a computer, for reading out a modification method from the modification method storage unit according to the selection of a surgical treatment, modifying the three-dimensional data related to the specification of the lesion by the selected method, and outputting the modified three-dimensional data.