1. A simulation system of cardiac function comprised of at least one computer to simulate a change in ventricular geometry by utilizing a cardiac structure model which represents a heart made of the myocardial tissue comprising both myocyte and connective tissue as continuum data of a composite material containing both matrix and reinforcement fiber, comprising: a material specification input part that inputs both connective tissue data representing mechanical property of the connective tissue within the myocardial tissue and myocyte data representing mechanical property of the myocyte within the myocardial tissue, and stores the connective tissue data and the myocite data into a memory; a geometry data input part that inputs geometry data of three-dimensional geometry of all or any part of the heart and stores the geometry data into the memory; a cardiac-structure-model construction part that generates, by a processor, the cardiac structure model that represents the three-dimensional geometry as an assembly of finite elements, sets a mechanical property and a direction of the reinforcement fiber with respect to at least one of the finite elements and sets a mechanical property of the matrix with respect to at least one of the finite elements, wherein the mechanical property represented by the myocyte data is set as a mechanical property of the matrix and the mechanical property represented by the connective tissue data is set as a mechanical property of the reinforcement fiber; and a simulation part that determines a boundary condition and calculates a change in geometry of the heart represented by the cardiac structure model under a pressure load according to the boundary condition.