Under the production to order environments, each completed product should be stored in the shop through the completion time to the due date of it (inventory duration of products). In this paper, the product inventory cost is defined as this duration multiplied by the holding cost per unit of time. An optimal algorithm to minimize the total inventory cost of products with sequence independent and dependent setup times is developed. An experience on computational time of the proposed algorithm indicates the effect by the variation of setup times and holding costs per unit of time. An application of the algorithm to a real problem is shown.