1. A system for assisting a user to decide what medical policy to undertake to achieve a target that has a cost associated with the target, and thereby minimize an actual cost associated with a user's decision, comprising: a computer processor having a data acquiring section and a function generator, wherein the data acquiring section acquires the target, behaviors which have already been adapted to the target, and the cost caused as a result of adapting the behaviors to the target as training data for generating a behavioral decision function as a function that determines a medical policy; and the function generator generates the behavioral decision function to minimize an expected shortfall of the cost to be obtained as a result of adapting the behaviors to the target, based on the training data, wherein the expected shortfall is determined with a probability of a generation of the cost exceeding a predetermined constant value, and wherein the function generator comprises: a first calculator that calculates the behavioral decision function by calculating the parameter to minimize an index value, which indicates an upper bound of the expected shortfall based upon a sum of costs exceeding value-at-risk in the training data, and that stores the calculated behavioral decision function in a memory, in the case that a provided value is the value-at-risk of the cost; a second calculator that reads the behavioral decision function calculated by the first calculator from the memory and that calculates a value-at-risk of a cost caused as a result of adapting the behavior shown by the behavioral decision function, based on the training data, thus providing the result to the first calculator; and a convergence judgment section that determines expected shortfall based on a convergence of the index value to a value within a predetermined range, wherein the upper bound of the expected shortfall is based on a second index value and minimized by the first calculator and wherein the upper bound is based upon the value-at-risk calculated by the second calculator; and the function generator outputs the parameter of the behavioral decision function calculated by the first calculator and the second calculator on a display device.