1. A simulation model defining system for automatically generating a simulation program for simulating a behavior of economy or society regarded as a system, a relation between constituent elements of the system being represented by a casual relation, comprising: a casual relation diagram defining unit for extracting, as nodes, specific items such as phenomenon, events and targets of said constituent elements, creating a casual relation by coupling said nodes with arcs, and setting metrics quantitatively representing a state amount to each of said nodes, to draw a casual relation diagram; a node table for storing a node name and drawing coordinates of each of said nodes; an arc table for storing node information on each of said nodes coupled to each of said arcs; a simulation model defining unit for adding a variable necessary for calculating said metrics to a simulation base model inheriting a structure of the casual relation of said casual relation diagram, explicitly writing a flow direction of information by extending a link between said metrics and said variable, and setting a calculation formula relative to said metrics and said variable, to define a simulation model; a variable table for storing information on a variable name, a variable type, a calculation model for identifying a calculation formula to be set, and the calculation formula, for each of said metrics and said variable; a link table for storing information on said variable connected to each of said link; a database for storing data of each of said tables and collectively managing the data; and a simulation program generating unit for automatically generating a simulation program in accordance with the data registered in said database.