To realize an operation control method for a private power generation system, which enables minimum cost operation by considering the unit cost of the purchased power and the estimated unit cost of generated power.

In this operation control method for a private power generation system which has a plurality of generators 4, when the load becomes two small if the number of generators, for example, is increased from one or the loads is two small for the two units of generators now in operation, the unit cost of the purchased power at that time is compared with the estimated unit cost of the generated power; if the unit cost of the purchased power is lower than the other, the quantity of power to be purchased is increased, and the number of generators in operation is kept to one or the two units in operation area reduced into one (chain 1). Power, the day demand is inputted in advance. And, in the case that to small a load is forecast, the quantity of power to be purchased is increased, and the number of generators is reduced before that. Likewise, when sudden drop in power load is predicted, the quantity of power to be purchased is increased to a quantity not to cause a reverse current, and the number of generators is reduced (chain 3).