1. A simulation system of cardiac function 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 to input 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; a geometry data input part to input geometry data of three-dimensional geometry of all or any part of the heart; a cardiac-structure-model construction part to generate the cardiac structure model that represents all or any part of the heart as an assembly of finite elements based on the continuum data of the three-dimensional geometry made of the composite material containing the matrix and the reinforcement fiber, and set the mechanical property represented by the myocyte data as a mechanical property of the matrix and set the mechanical property represented by the connective tissue data as a mechanical property of the reinforcement fiber; and a simulation part to calculate a change in geometry of the heart represented by the cardiac structure model under a pressure load by utilizing finite element method with computation.