An in-vehicle software updating method includes: acquiring, after starting a software update, a voltage measurement value of an in-vehicle power source; and acquiring a progress rate in the software update in a case in which the voltage measurement value is equal to or less than a second threshold. In a case in which the progress rate is less than a set value, the software update is interrupted. In a case in which the progress rate is equal to or greater than the set value, an operation mode of the zone control unit is switched to a power saving mode, and a second predicted voltage value of the in-vehicle power source at the update completion time is calculated. The software update is continued in a case in which the second predicted voltage value is greater than a first threshold.