1. An electric demand/supply planning apparatus, comprising: an initial demand/supply plan creating section which creates an initial demand/supply plan of a controllable electric power source in an electric power system into which a natural energy electric power source is introduced, based on a demand predicted value, a predicted value of a natural energy electric power source power generation output amount, and electric power source equipment data; a demand/power generation amount probability density distribution creating section which creates a probability density distribution of a demand and a probability density distribution of a natural energy electric power source power generation output amount at every predetermined processing time, by using fluctuation bands of the demand and of the natural energy electric power source power generation output amount which are set in advance; an economic load dispatch calculating section which calculates an output allocation to minimize a total fuel cost of the controllable electric power source based on the initial demand/supply plan created in the initial demand/supply plan creating section, the probability density distribution of the demand and the probability density distribution of the natural energy electric power source power generation output amount which are created in the demand/power generation amount probability density distribution creating section, thereby to create a prediction distribution of a power generation output of the controllable electric power source; and a display section which displays the prediction distribution of the power generation output of the controllable electric power source created in the economic load dispatch calculating section.