When a smartphone, which is an operation terminal connectable to the Internet, receives input for change in hot-water set temperature of a hot water supply device having a hot-water supply function, and the hot-water set temperature after the change instructed by the input for change is within a predetermined high-temperature region, then a request for input of a password is generated for the smartphone. When a password input to the smartphone matches with a predetermined registered password, an instruction for change in hot-water set temperature generated in accordance with the input for change is input to the hot water supply device via a communication adapter.