1. A simulation model defining system for generating a simulation program for a simulator simulating a behavior of economy or society regarded as a system of phenomena and events, a relation between constituent elements of the system being represented by a causal relation, comprising: a causal relation diagram defining unit, for representing a user interface screen of a causal relation among phenomena and events structured into nodes and arcs of a causal relation diagram, to receive definitions of nodes and arcs and settings of metrics from a user, and storing metrics information of a node name, node information and drawing information of each of the nodes and arc information of arcs each combining an effect node and a cause node into a database; a simulation model storing unit, for representing a user interface screen of a simulation base model inheriting a structure of the causal relation of said causal relation diagram for input from said user of a variable necessary for calculating said metrics to the simulation base model and setting a calculation formula relative to said metrics and said variable, to receive at least one of a rate variable representative of a flow amount of articles or information per unit time, a level variable representative of an accumulation amount of flow rates, a constant, a variable object, and a link object representative of a flow direction of information, a selection of a calculation model for determining a calculation method for the variable, definition contents for setting a specific calculation formula for the variable, and a selection of a graph function to define as a calculation formula and numerical value plotted between an input variable and an output variable from said user, so as to display variable objects and link objects inputted at the simulation model definition screen and to store a simulation model in a variable table and a link table of the database; and a simulation program generating unit, for reading simulation model data from the database in response to receiving a simulation model name, a simulation period and a time bucket of simulation received, and if a variable type is a level variable, extracting cause and effect variable names corresponding to the level variable to generate a simulation program for a template including an equation for calculating a value of a next simulation step, and if a variable type is not a level variable and if a calculation model is coincident with a calculation formula, generating a simulation program by using a template including of an equation, a variable of a calculation formula, and definition content of a variable, and if a variable type is not a level variable and if a calculation model is a graph function, reading simulation model data defining a graph of the variable from the database to generate a simulation program for a template constituted of an input variable and an output variable for the graph function.