An in-vehicle software updating method for updating software in an in-vehicle system, the in-vehicle system comprising a zone control unit (12; 12A) configured to manage a plurality of control targets (13, 14) connected on a downstream side of the zone control unit (12; 12A), the in-vehicle software updating method comprising: retaining update data in a first area allocated in advance in a memory of the zone control unit (12; 12A); calculating, before starting software update using the update data, a first predicted voltage value, Vp, of an in-vehicle power source at an update completion time, t2; starting the software update in a case in which the first predicted voltage value is greater than a first threshold VL1 (S12: YES); and acquiring, after starting the software update, a voltage measurement value, Vx, of the in-vehicle power source; characterized in that the method further comprises steps of: acquiring a progress rate, Rx, in the software update in a case in which the software update is not completed (S15: NO) and the voltage measurement value is equal to or less than a second threshold, VL2, (S16: YES), the second threshold being greater than the first threshold; interrupting the software update in a case in which the progress rate is less than a set value, R1, (S17: NO); switching an operation mode of the zone control unit (12; 12A) to a power saving mode in a case in which the progress rate is equal to or greater than the set value (S17: YES), calculating a second predicted voltage value, Vp2, of the in-vehicle power source at the update completion time, interrupting the software update in a case in which the second predicted voltage value Vp2 is equal to or less than the first threshold (S20: NO); and continuing the software update in a case in which the second predicted voltage value is greater than the first threshold (S20: YES). retaining update data in a first area allocated in advance in a memory of the zone control unit (12; 12A); calculating, before starting software update using the update data, a first predicted voltage value, Vp, of an in-vehicle power source at an update completion time, t2; starting the software update in a case in which the first predicted voltage value is greater than a first threshold VL1 (S12: YES); and acquiring, after starting the software update, a voltage measurement value, Vx, of the in-vehicle power source; characterized in that the method further comprises steps of: acquiring a progress rate, Rx, in the software update in a case in which the software update is not completed (S15: NO) and the voltage measurement value is equal to or less than a second threshold, VL2, (S16: YES), the second threshold being greater than the first threshold; interrupting the software update in a case in which the progress rate is less than a set value, R1, (S17: NO); switching an operation mode of the zone control unit (12; 12A) to a power saving mode in a case in which the progress rate is equal to or greater than the set value (S17: YES), calculating a second predicted voltage value, Vp2, of the in-vehicle power source at the update completion time, interrupting the software update in a case in which the second predicted voltage value Vp2 is equal to or less than the first threshold (S20: NO); and continuing the software update in a case in which the second predicted voltage value is greater than the first threshold (S20: YES). acquiring a progress rate, Rx, in the software update in a case in which the software update is not completed (S15: NO) and the voltage measurement value is equal to or less than a second threshold, VL2, (S16: YES), the second threshold being greater than the first threshold; interrupting the software update in a case in which the progress rate is less than a set value, R1, (S17: NO); switching an operation mode of the zone control unit (12; 12A) to a power saving mode in a case in which the progress rate is equal to or greater than the set value (S17: YES), calculating a second predicted voltage value, Vp2, of the in-vehicle power source at the update completion time, interrupting the software update in a case in which the second predicted voltage value Vp2 is equal to or less than the first threshold (S20: NO); and continuing the software update in a case in which the second predicted voltage value is greater than the first threshold (S20: YES).