An updating method for in-vehicle software for updating a software included in an in-vehicle system having an uppermost control unit, a zone control unit connected to a downstream side of the uppermost control unit, and a plurality of lower control units (13, 14) connected to a downstream side of the zone control unit, the update method comprising: securing one area in a memory on the zone control unit, the one area being capable of storing one update data (31, 32, 33) for updating the zone control unit and the lower control units (13, 14) which are an update object, wherein neither of the lower control units (13, 14) includes a memory area for update; the update method further comprises the following steps to be performed by the zone control unit: repeatedly checking whether the update data (31, 32, 33) exists in a predetermined software supply source provided in a cloud outside the in-vehicle system; downloading the update data (31, 32, 33) from the software supply source to store the update data (31, 32, 33) in the one area, when the update data (31, 32, 33) satisfying one download condition (S15 to S19) defining a download priority of the update data (31, 32, 33) exists in the software supply source; selecting the one update data (31, 32, 33) with the highest priority from a plurality of update data (31, 32, 33) to execute the downloading, when the plurality of update data (31, 32, 33) satisfying the one download condition (S15 to S19) exist concurrently in the software supply source; executing the checking and the downloading of the update data (31, 32, 33) to store the update data (31, 32, 33) in the one area, when an ignition signal, SG-IG, of a vehicle is in an on-state; updating the software of the update object by the update data (31, 32, 33) stored in the one area, after the ignition signal, SG-IG, is switched to an off-state; and releasing the one area by the zone control unit when update installation is normally completed. securing one area in a memory on the zone control unit, the one area being capable of storing one update data (31, 32, 33) for updating the zone control unit and the lower control units (13, 14) which are an update object, wherein neither of the lower control units (13, 14) includes a memory area for update; the update method further comprises the following steps to be performed by the zone control unit: repeatedly checking whether the update data (31, 32, 33) exists in a predetermined software supply source provided in a cloud outside the in-vehicle system; downloading the update data (31, 32, 33) from the software supply source to store the update data (31, 32, 33) in the one area, when the update data (31, 32, 33) satisfying one download condition (S15 to S19) defining a download priority of the update data (31, 32, 33) exists in the software supply source; selecting the one update data (31, 32, 33) with the highest priority from a plurality of update data (31, 32, 33) to execute the downloading, when the plurality of update data (31, 32, 33) satisfying the one download condition (S15 to S19) exist concurrently in the software supply source; executing the checking and the downloading of the update data (31, 32, 33) to store the update data (31, 32, 33) in the one area, when an ignition signal, SG-IG, of a vehicle is in an on-state; repeatedly checking whether the update data (31, 32, 33) exists in a predetermined software supply source provided in a cloud outside the in-vehicle system; downloading the update data (31, 32, 33) from the software supply source to store the update data (31, 32, 33) in the one area, when the update data (31, 32, 33) satisfying one download condition (S15 to S19) defining a download priority of the update data (31, 32, 33) exists in the software supply source; selecting the one update data (31, 32, 33) with the highest priority from a plurality of update data (31, 32, 33) to execute the downloading, when the plurality of update data (31, 32, 33) satisfying the one download condition (S15 to S19) exist concurrently in the software supply source; executing the checking and the downloading of the update data (31, 32, 33) to store the update data (31, 32, 33) in the one area, when an ignition signal, SG-IG, of a vehicle is in an on-state; updating the software of the update object by the update data (31, 32, 33) stored in the one area, after the ignition signal, SG-IG, is switched to an off-state; and releasing the one area by the zone control unit when update installation is normally completed.