The distribution center became more important because the growth of the internet enabled us to buy various products easily on the Internet. In the distribution center, one of the main works is order picking which is to carry the products from shelves of the warehouse according to the orders received from customers. Efficient order picking reduces the large amount of work and the improvement of the order picking is necessary in a larger distribution center. It is important for order picking not only to give workers a good tour plan but also to assign products to the storage shelves in the distribution center. It is known that it is effective to consider frequency order and frequency of co-occurrence at the same time for storage location assignment. The conventional studies proposes the class-based method for frequency order and evaluation function to evaluate frequency order and co-occurrence of orders at the same time. This study proposes a novel method how to assign products to the storage using a co-occurrence network of ordered products and self-organizing map on network topology of the warehouse. Our proposed method is compared with the conventional studies and our experiments show that our proposed method can be more effective than the conventional methods.