To provide a production planning method capable of facilitating measures to a shipping request without any delay, and minimizing stock quantity and an inventory space.

In this production method for a mixed flow production line for continuously producing first products A to be mass produced and second products B to be produced in a low volume in a mixed flow state for periodically shipping the prescribed quantity of products, the assignment of the production quantity per unit time of the first and second products A and B is instructed based on the product shipping statistics of the first and second products A and B each time in the past relatively long period, and the stock quantity (n) of the second products B to be produced in the small volume is set so as to be not less than the maximum quantity N of the second products B occupying in one time shipping quantity of the product shipping statistics, and so as to be less than (N+α) (α is a conventional excess quantity). At the time of shipping the second products from the inventory, the assignment of the production quantity is shifted to the second product B side corresponding to the shipping quantity.