The best-before date for cereal I purchased had expired. I would like to return the product for a refund as well as pursue a penalty for the retailer.

The "best-before date" is the date by which the product will remain good until the date indicated when the bag or container is not opened and the product is stored following the storage instructions. In Japan, there is no legal stipulation that imposes criminal penalties for selling food products passed their best-before dates. However, since retailers are expected to sell foods before the best-before date, you should promptly bring the actual product and the receipt to the retailer where you purchased the cereal, inform them that the best-before date has expired, and ask for the product to be returned and refunded or replaced with a new product.