It used to be difficult to automatically mix--together with ordinary pet foods--nutrients and medications as additives based on a prescription from an animal hospital according to the condition of a pet. Also, it was not easy to submit from a personal computer or a mobile phone an order for custom-mixed pet food. This system easily receives, via the Internet from a personal computer 1 or a mobile phone 2, an order for custom-mixed pet food based on a prescription sent to a custom-mixed-pet-food prescription control computer system 4 that electronically controls a mixing device 5 that automatically produces custom-mixed pet food according to mixing data specified by an pet owner's order and prescription from an animal hospital. The system comprises a pet-owner-user's personal computer 1, a mobile phone 2, the Internet 9, a custom-mixed-pet-food prescription control computer system 4, a computer 3 of an animal hospital, a Web server 4a, an application server 4b, a database server 4c, a stored data input/output processing management terminal 4d, an interface circuit 4e, and a mixing device 5.