This paper analyzes production costs under a production planning to meet demands. These costs are divided into two categories : "additional cost" and "basic cost". The latter depends on technical levels and intensity of labour etc. The former, which involves overtime-undertime cost and inventory holding cost etc., represents the effectiveness of production planning.It is shown that the criterion to minimize total costs is equivelent to minimize additional costs on condition that the basic costs are deterministic. The additional cost function of overtime-undertime is newly formalized, excluding a contradiction of the usual formulation of overtime-undertime costs.