To provide a prepaid-system gas supply control system in which device cost is reduced without needing provision of a new communication line at the time of installation.

A home display device 3 connected to a gas meter 2 is installed in each gas consumption place 6. When transmitting a gas purchase request to a server 5 from a communication terminal 12 such as a cellphone and performing a money payment procedure, the server 5 performs settlement processing of a gas purchase price, and issues a rewriting code effective in only the home display device 3 storing and holding an in-subtraction-counter-start point based on the in-subtraction-counter-start point and a gas purchase amount received from the communication terminal 12. The home display device 3 collates a generated collation code with the input rewriting code based on the stored in-subtraction-counter-start point, decides whether or not it is an effective rewriting code, and adds the gas purchase amount to a subtraction counter 7 when it is effective. At that time, the in-subtraction-counter-start point is updated to a value different from before the addition.